Forza 3 was a little overrated in a lot of reviews. Even ISR admited they rated Forza 3 more than they should after they spend more time with the game. Gt5 reviews may be a little lower , even lower than GT5P in some cases.
The problem with sims reviews is often you have to spend a lot of hours to get a feel where it stands compared to other games which can be finished in a few hours.