In itch, when you click the download button, before generating the download link, Itch asks you if you want to donate money to the game.
If your game has downloadable content, it only shows you the download button, because it does both functions.
If your game only works on the web, then the same button, instead of saying "download", will say "Support this game"
The page layout is dependent on screen size. The responsible css changes stuff. I do not know why they made two locations for the download/donate/buy button. But there are scenarious where you can change size of the page and suddenly you have two buttons instead of one or vice versa. I believe this happens for free games without donations.
So my guess, they repurposed one of the buttons, the upper one, to do multiple stuff and the name of the button is not always good. The download button on the bottom of the description seems to be the main one, but they also have to allow for pure html5 games that could be donated to, instead of downloaded and do not have the main download button.
