Remember that review can be different due to the actual reviewers themselves.
The GT5 IGN review was not done by a certain Chris Roper, fan of the series, and former member of IGN. Chris reviewed GT4 and gave it a very high score - 9.5. In the GT4 review, he states that "There is the nagging feeling that developer Polyphony Digital is refusing to expand its own world..". This "problem" has been echoed by reviewers of GT5, and unlike other reviewers, Chris, as a fan of the series, has the ability to fully comprehend Polyphony Digital's "different" ways of making video games, and not let this alter his review of the game. Other reviewers may not understand the "different" way that PD works, and mark the game worse as a result.
Personally, I believe that although many points raised in the various reviews are true, a fan that understands the magical world of Gran Turismo; and is able to overlook the minor flaws, and that it isn't "Need for Speed" and that cars don't explode into a million pieces upon impact, that they will have lots and lots of fun with the game for years to come.
[joke] BTW, when is the Gran Turismo 6 release date again? I hope we see a trailer at E3 next year! [/joke]
I have actually sent a Facebook message to a certain Christopher Lee Roper asking him his opinion on IGN's GT5 review and I will post his reaction when I get it (although I did mention to him that he would be free to answer after playing the game thoroughly, so don't hold your breath)
Personally even though I haven't played the game, I was expecting IGN to give it I higher overall score but personally I think that its more important to look past whatever flaws that may come up and enjoy the game.