Some games are heavily hyped by publishers, developers, and even console makers. When a reviewer is constantly exposed to spin and positive statements about a game, it colors his impressions dramatically. The corresponding review scores are accordingly inflated. The game's marketing affects what is supposed to be an objective measure of a game's quality. We buy the game largely as a result of these impressions and reviews and only realize after the marketing has subsided that we may have been duped.
Games journalism, in its current state, is largely a shill for moving product. It's either an industry that is so reliant on staying in the good graces of publishers that they sacrifice journalistic integrity, or phlegmatic enough that it is an unwitting branch of a publisher's PR.