Obviously those better quality ones are render (very easy to tell), basically they are artworks, the others are realtime.
To look a lot better Forza need to redo the lighting settings and the colour palette, some users do great picture where Forza looks much more real, if we could only tune the general image like that. I hope Turn10 does it in Forza 4, just improving that will be a world of diference since the cars and the tracks are very well made.
As for the E3 2010 presentation, all kinect stage demo were faked, not because the games/tech demo didnt exist, it was because MS didnt want screw up in stage like the last year, Forza and other were showed working behind closed doors to the press.