Certain reviewers giving games low scores specifically to grab attention on metacritic are nothing special. It happens all the time with exclusives. Don't think that this case with GT5 is anything special. In fact, there may be lower scores still to come, who knows, this may be a legitimate (albiet low) score. For example, gameshark gave Reach a 5/10 but that doesn't really bring down the average much since there are almsot a hundred reviews. The higher scores from "loyal" sites helps balance it out, (lets face it, these exist for both the 360 and the PS3). This case with GT5 is not special, PS3 fans are not being shortchanged.
Typical excuses to justify low scores (or unmet expectations):
Reviewers are teh baise!
Microsoft is paying everyone else off.
This game is the second (or even third!) coming. Don't heed the scores, game is perfect!.