I dont know how GTA5 manages to be on this list every week; i believe it has reached record sales numbers by now.

Huge fan base and free continual updates. I wouldn't be surprised if people bought, sold and re-bought it after a big DLC update.

So uh... what's with No Man's Sky? I understand it is 60% off but that doesn't seem like enough to put it in the top 10 sales. EDIT: Apparently there's a new update that's supposed to address pretty much every major complaint people had with the game, including multiplayer, not enough variety, and not enough things to do. Sounds interesting. Recent reviews on Steam seem to be relatively good.