Xbox fans on video game forums like to brag about Forza's meta scores but the userbase overall didn't show the love with their wallets. Sure it sold good overall but Forza's been bundled heavily this whole gen and needs to be forced on consumers to get it to sell it's ltd numbers. GT gets bundled a lot too but it doesn't need the bundle to sell it's initial 5-6 million on sales anyways.
Following the weekly charts on this site since 2007 I can tell you that Forza usually stalls at about 2.5 - 2.8 million before bundles carry it to 5M or so. It's a great series and a great alternative to GT for xbox users but it will never pass GT in sales.
The same reason why Killzone or any other FPS that Sony creates will never outsell Halo. The name brands are set in hearts and minds and a couple thousand video game forum users can't change that for either side.
