To be honest, if I see any game get a 10/10 score, I'm sceptical about it, because I think a game can only get a 10/10 score if it's based on a first impression, and not on the entirety of the game. In fact, most times where I've read 'perfect' reviews, the reviewer freely admits that it's based on only the first few hours of gameplay. Mind you, I'm specifically talking about perfect scores that have been given before a game is officially released, which is when those scores are most important. Plus, I've been let down before. (FFXII did not deserve the perfect scores it got; I liked it even less than X-2...)
I don't think it has anything to do with fanboyism at all. I think it depends on how good game developers are at marketing games to reviewers, because that's their job; they're supposed to sell their product. They'll select certain parts of their game to show to reviewers that colour their expectations in a positive light. If game developers don't try particularly hard to sell their game to reviewers, and just throw them into the first level, then maybe they do lose 4% in the overall score.
Either way, it doesn't really matter. I've learned that the numbers that come out before a game is released to the public aren't always the best representation of how good a game is.
