You need to check every game you compare online for info. Also make sure to check the graphics mode on the game, performance vs quality. In general, it's rare for the same game to have the same settings on both platforms, and that's not even taking into account dynamic resolution.

For a good check take something that is confirmed to be the same, eg Quake 1, set it to the same graphics settings and then take screenshots of the exact same spot, eg main menu, level start, etc and compare those in some app pixel by pixel, praying the screenshot compression won't affect your results 😉.