Hi,
The issue comes from the customization done to your product page layout.
If you look at the HTML of a product page, you can see that:
monosnap.com/file/VY7I7xQx1qXUIg0KnFaR6MRraBoujf
The hidden data of the variants of the product are inside the form of the product page where they should normally be outside.
Because of that, when you click on add to cart, instead of adding the selected variant, it adds the last one to the cart.
You're probably missing one or two </div> in the show_tabular view file where the customization was done.
If you didn't customize the product page layout and that this comes from your template, then you want to check on that with your template provider so that they can provide the appropriate fix.