I think it's a psychological thing, in that, if it's vertically mounnted and you play a horizontal game, you notice what's above and below. Plus you have to look higher in some games than you would for vertical games in that orientation. However, mounted horizontally, you're looking in the same place you would when playing vertical games in that orientation. And, vertical games on a horizontally mounted 27" are about the same size as the original 19" vertical monitor games. (Slightly larger, actually - although I use 5:4 aspect, which makes 'em a little broader, even).