I think its kinda messed up that you can generally safely assume some games will get good scores, like a GTA or FF game, at least from the major sites like IGN. Meanwhile a thought like, of course Mirrors Edge will get a 9, never crossed my mind.
Although maybe the problem is since we assume AAA games will get scores higher than they deserve, we are extra critical of those games. So going in to playing Halo 3, the attitude is already 'yeah this is a pretty decent game, but its surely not a 9.5!!'. I mean, its pretty cool sometimes to be that guy who has the erudite criticisms of the games everybody else loves.