People are generally more dominant in one eye (usually the right) rather than the other. By having an offset sight, the pilot would not have to lean in order to align his dominant eye with the gun sight. This doesn't really give either a centered or offset gun sight an advantage, it's just another way to do things.
It seems more awkward in the game, as your monitor is only a single point of view, as opposed to two MkI eyeballs.