This is happening because you are using “Individually priced files” to set your product price. Please read our guide here:

https://itch.io/docs/creators/pricing#purchase-tiers-through-individually-priced-files

Use individually priced files for quick bonus content, not to set the price of your content. Use the minimum price on your project to set the price of your product, as this will allow you to change the price at a later time without affecting ownership.

We strongly advise against using individually priced files. If you have a free file for a page that is otherwise paid, select the “Demo” option, and set the product price of the page.

If you have a free file for a page that is otherwise paid, select the “Demo” option, and set the product price of the page

The mindset is often the opposite. You have a free game with optional paid items. A demo version is not the same as a free public version. Demo implies that it is not the full game and seriously restricted. Whereas a free "public" version would be the full game and there might be additional or bonus content.

It might be because this is seen on Patreon so often. You get more, the more you pay, but the basic thing is complete and not a mere demo.

It also is a difference if you will appear in paid or in free games. And if a dev created a free game, why should that dev want to put that game in the paid section of browse, just because there are some things that can be bought optionally.

Individually priced files sound like they would solve this, but they do have issues. Creating multiple project pages for the same project is also discouraged, but apparantly the only way to deal with those issues.