You have to realize professional reviewers are coming from a completely different place than gamers; that's why the discrepancies exist (along with natural variation). They play hundreds of games a year, are aware of the best and the worst titles, have little interest in length, and are simply looking to get the most out of they time they spend playing. Their perspective is radically different from someone who is looking for a value buy, or a title to hold them over before new releases, etc. They do their best to judge absolute quality impartially, but they don't always succeed because they aren't like us by lieu of the very thing they're doing. But they are very likable people in every instance where I've actually heard them talk.
Check out the podcasts on Giant Bomb, Game Informer, IGN, etc. It adds a human side to things and makes me seriously doubt all of the accusations of outright corruption.
EDIT: Having said that, two of my favorite games from this year so far have been Deadly Premonition and Alpha Protocol: they're hardly critical darlings. So I really don't always agree with them either.
p.s. I hate Gears of War.