What's so shady about it? There are differences between a preview and a review, with the biggest difference being the fact that the former was likely formed by only playing a portion of the game while the latter is the result of experiencing the final or near final product in its entirety.
A lot can change between those two experiences. Maybe the game is incredibly fun in short bursts, and at the forefront of your time with it, but its funfactor wears thin the longer you play it. Maybe the one or two modes you did play were ridiculously fun, but once the final product came into your possession, you came to the realization that feature depth (modes, maps, weapons, etc) were lacking. These are things that can be hidden in a preview, artificially inflating a game, that wouldn't be known until you have considerable time with the final product.
A lot of people in this thread are just out for blood, but a lot can change between a preview and a review. Also, I agree that a 100 point scale is stupid for media reviews, but there is a big difference between an 8.9 and a 9. I bet everyone here would do everything in their power to get their 89% in a class bumped up to a 90% The same thing applies here.