Why do my users 're'select the amount in the cart?

Maybe because they are curious about what is inside this combobox's arrow.

Have you tried to change it to another component like a spinner ?

I don't have any particular research about it, it is just my guess.

From a non-UX designer perspective:

It looks like your €2.499,00 is crossed out while being replaced and highlighted by €650,00. This would be jarring to anyone who was expecting and prepared to pay €2.500,00, but then seeing a price that is considerably lower. If I could save €1.849,00 on a purchase I would definitely do everything I could think of to make sure that happens.

Of course, your €1.849,00 figure comes from €2.499,00 - €650,00, but €1.849,00 also happens to be almost 3x the €650,00 amount. So they may be clicking on the quantity expecting it to 'realize' it's trying to say 3 but was glitched/mistaken into saying 1.

One note: people may be doing this kind of action without really thinking about it, because they know whatever happens prior to payment is low-risk...you can click all the buttons, in any order you want, and there is no risk until you actually hit that final 'Submit payment' button. Worst case scenario, you empty the cart and start over. So even though it's easy to calculate 650*3=1950, people might just be thinking 6xx*3 (dealing with only the most significant digit) = 18xx (since 6*3=18). If they continue with checkout, then they eventually figure it out, but it's that initial, split-second reaction that might be causing them to check the quantity.

Instead of crossing out the €2.499,00 figure entirely, I'd leave it as normal text, and show that the €650,00 is subtractive (since it is a discount, after all). I don't know about localization, but usually just putting a - in front of the number would suffice. Then it's more obvious where that €1.849,00 figure is coming from. And while it's nice to highlight discounts, you also highlight the total discount down below - so you might want to play around with leaving the text in the Voordeel column as just plain black text.

Also in the lower right, the Subtotaal and Totaal being the same is a bit confusing. Should the Subtotaal be the sum of all Adviesprijs? I'd personally expect this layout:

Subtotaal (sum of all Adviesprijs)
Uw totale voordeel (sum of all Voordeel)
Totaal (Subtotaal - Uw totale voordeel)

If you can, perform some testing and gather information about why are users clicking on the amount dropdown. Probably most users are clicking for the same reason.

That said, my guess is that it might not be absolutely clear that what there is behind the dropdown are plain numbers:

1
2
3
...

and users might think there could be other kind of options:

1
1 without X Pakket
2 with X Pakket
3 for 2
...

The dropdown you have is used to select between different options. And although each amount is an option, possibly this dropdown is not the best component in this case.

This is the dropdown from Amazon:

and from Google Store:

which, in my opinion, are quite similar to the way most browsers display the number input :

(My first thought was that users might want to check how much would 2 elements cost, but this is not the case as you say after opening the dropdown option 1 is selected.)

Another possibility, given the age of your users: All through the 1990s and early "aughts" a light gray like you are using meant "not an option". We called it "grayed out". So, to me, (I am in your user's age group) it looks like it's not active for some reason.

Also, you have one bright color on what I think is the discount rather than the price that is to be paid. I find that confusing, so I might adjust the quantity (my only real option) just to get a better sense of what I am looking at.

I just managed to find your website and proceeded as if I was a customer. And maybe with a little bias, but I also clicked the 'amount' button out of nowhere.

It is hard to explain, but it is just a little 'tic'; as you are offering quite expensive products, the user wants to check that everything is 100% set up right (clicking it and verifying it's set to one gives the user a safe impression). The button in question is also the only button the user can click that does not ignite an action:

All other buttons will somehow change the product settings in one click and/or redirect the user to another page.

All in all; I think it's just a little piece of automatism of the clients checking their orders, I wouldn't recommend changing the button.

The "1" in grey on grey looks "disabled". Grey means "disabled". So some of your users probably think that the quantity is not set yet and click to set the quantity to 1.

The quantity is not the only problem. On the images, I see an accessibility problem. Several meaningful texts and figures are in grey on white or in grey on grey. 1) This may mean "disabled". 2) This is difficult to read.

Improving that would be a good idea.

I would like to add something from my perspective. Aged 36 I could fall into your target group.

I don't know about localization in your case, but living in Baltics my order of thought and reading from left to right is Price - Discount = Price I pay and only then Quantity I buy.

In your case it does not mach my order, that makes me "uncomfortable" and I start to double check if my thoughts are in line of what webshop is showing and trying to sell me.

Another point would be How often people buy more than one item priced at 1849 EUR? Would that make quantity selector in this case obsolete?

I am one of the users who often messes with the quantity even though I will probably buy only 1 item.

When I get to a checkout page, it is common for me to be curious what the maximum value is in a quantity drop-down list, and I am even more interested in any possible discounts I might get if I purchase more than one. For example: On some websites, if I change the quantity to 2, or better yet to a higher number (5, 10, more) the price sometimes comes down. These "bulk discounts" are often listed in the item description, but not always. After checking to see what changes are made by my other quantity checks, I usually change it back to 1.

Another reason why I personally would select an item from the quantity list, even if only to select 1 when 1 is already selected, is to attempt to force an update on the page. If I changed some other option on the page but did not see the price change, I might wonder if the selection did not force an update of the other data. Looking at the screenshot posted in the answer from @Thomas, I can see you have, at the bottom, what appears to be a shipping selection. If I changed the shipping option and the 1.849,00 amount did not change, then I would click next on the amount and select 1 (even if 1 is what was already selected). If that also did not force a page update, I might look for another control on the page which I thought might force an update. In the end, if the 1.849,00 amount never changed, I would just think "Maybe that is not the total. Maybe there will be another page with a total on it." and I would continue. This behavior of mine would extend also to any other options you have that I might change.

These are common actions I perform whenever I am performing an online purchase checkout.

When I see the one downward pointing arrow, it feels like a drop down menu where you have to make a choice before proceeding. The other options make it more clear that there is already a default option selected.

They are clicking on it because there is an arrow there and they are curious what the arrow does.

If there was an up-arrow and a down-arrow, they would realize that the arrows changed the value, but as it is, they think there might be a useful menu there.

As @nigel222 suggests, the button or link "Remove" may be missing.

Is the quantity "0" present in the list? I don't think so. So, indeed, the users who puzzle @JaneDoe1337 may be trying to remove the item from the basket. Idea: they want "0", so they unroll the list, but they don't find "0" in the list.