I would guess, the game Ark is not just rendering a scene but all the other parts like CPU is also used. And the game logic itself tries to balance its usage or GPU is limited by the CPU or RAM speed in example. While the menus you was talking might not have any fps limits, and doesn't use anything other than the GPU (for the most part) and renders as much as it possible can. Edit: OK I didn't read properly... So the 80% of Ark is in the menu too? Well maybe the game has builtin limits in the menu, and CS:GO does not.

BTW for that reason I recommend fps limits or any sort of VSync. Some games allow limiting the framerate in menus separately from the ingame. If I'm not mistaken Marvel Rivals is such a game that allows that.