It has everything to do with expectations. For contemporary example, why do you think Assassin's Creed Brotherhood is garnering 10's? It's because everybody expected it to be a shitty add on and it actually seems a very decent improvement but without doing anything drastically different and it was done in under a year.
Same with the Forza series to an extent. It is the largely seen as the underdog and as such there are lower expectations surrounding the franchise (or at least there was previously) and the fact that they put out 2 very good and very polished games probably stunned a lot of games journos and it earned very, very high scores in across the board because of the extent to which it surpassed expectations of the franchise.
GT5 would've had to have (hyperbole warning
walked on water to get the same same review scores across the board as Forza did. It's still garnered excellent reviews from some quarters, and indeed from a large majority of those who bought the game. It was never going to get high reviews from the likes of IGN though, who seemingly rate pretty menus as equally important to a driving game as having a stunning, feature rich and currently unmatched driving component.