One thing to note with that chart - The huge spike on Forza 3 sales after Week 50 came about due to the Holiday bundle. Granted, GT has had its fair share of bundling too (see the bump after Week 100) but nothing to the extent of that Holiday bundle which added what, a Million to the F3 total?

I wouldnt be so sure about that. 

Because Gran Turismo is the real driving simulator, duh.

Total BS. It's the only real sim racer on console and is full of content. It appeals to car nuts like forza simply doesn't. Gt5 was a mixed bag but PD kept updating and tweaking it for nearly 2 years, that kind of dedicated support is rare. The little details that car nuts love, you simply cannot get anywhere else.

In other news Michael Jackson is the king of pop, Disneyland is literally the happiest place on earth and everyone in MacDonalds is loving it.

To be honest I wouldn't be surprised if the gap goes back into the PS2/OG Xbox realms come GT7's release. I know a lot of people simply just gave in and gave FM2 or 3 a shot after becoming fed up with GT5's ambiguous development process.

Perfect Dark, Doom, Unreal etc. vs Halo?

This!

Much the same like say:

Halo, iPhone, MCDonalds, Ben & Jerry's and everything else you've tried before. There are no mysteries here.

Plus distribution of sales (from VGC) according to pd is wrong as they have gt5 at 2.8mill for North America. Plus Gt is at least 200k under tracked imo

Cause it is the only real driving simulator for a hardcore community(on consoles)
The whole game is aiming to turn you into a good driver and people like that focus.
And of course cause of Playstation and Europe.We like quality and driving.
We even have the GT Academy TV Show on TV.