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.