Part of it is just legacy/expectations. Reviewers fall victim to this in a big, big way. If a game is from a company that is beloved for producing great games, and the game is highly anticipated, then it gets stellar reviews no matter what the case. The most recent example of this is Civ 5. It is not a bad game, don't get me wrong, it got my $50. However it is not the 10/10 that most sites gave it. Compared to Civ 4 it is lacking in some key ways. It needs some work to be as good. However it was anticipated and Firaxis is beloved so it got through the roof ratings. If you go read user reviews, and some of the less "mainstream" reviews you'll find that indeed it is a good game, with some flaws that need fixing and it gets the more 7-8/10 that it should.
Valve would be another great example of this. There seems to be nothing those guys can release that every reviewer doesn't claim to be the best thing EVAR. Again, it is a case of them wanting it to be great. Because Half Life was such a cool, genre defining game, now everything they do is rated as wonderful, no matter what. A fairly recent example would be Left 4 Dead 2. The original L4D was a fun game, but I don't think quite worthy of its universal praise. However L4D2 was really nothing but a cash in. Released a year later, almost nothing new, it was an expansion back in all but name. Not bad, but not worth $50. However it again got universal acclaim, because it was from Valve. Had it been form someone else, it probably would have been looked down upon as a ripoff.
Reviews are more useful in the case of less anticipated releases. When it is just a game that is coming out from a developer that isn't really special, they often are far more accurate. They evaluate what the game is, not what they want it to be.
However some people, Valve, Blizzard, Rockstar, etc, just don't get real reviews. Their reputation influences things and so the reviewers inflate things without realizing it. The have mentally prepared themselves for the game to be awesome, and so it is to them even if it really isn't.