Hey! You're definitely not stupid. 😄
Depending on when you originally bought it, this chapter has had 3–4 updates over the past two years. I also sometimes add new renders to the page/gallery to show what's coming in the next update, and occasionally add bonus galleries too. itch.io sends notifications for those as well.
You don't need to pay again for every update. Once you've purchased the game, the updates are included with your purchase.
I think you may have been downloading the wrong file or possibly interpreting the page/gallery notifications as a new downloadable version. The actual downloadable version is updated on the game page whenever there's a new release.