If you're evaluating games from screenshots or videos, and can take the time to evaluate minute differences between the two systems, you can see a graphical difference. Regardless of whether a game is multiplatform or not, when you're playing the game on either platform the differences will be small enough that you won't notice them because you're preoccupied with what is happening in game.
Another way to look at this is the following:
Human beings do not really see or hear things the way you probably think they do ... Everything we see, hear, taste, smell or feel is our brain's interpretation and "best estimate" of what is really happening. As such, when we're playing a game our brain focuses on the action that is happening on screen and interprets it as if it was a real environment; unless something catches your attention for being "unrealistic" or poorly done, you're probably not going to notice it and you're going to accept the environment as being "real" ...
To a certain extent this explains why when you go back to a 10 year old game the graphics seem far worse than you remember, and it can (often) be less engrossing.
