First many of the shots are post stamp size which makes it look better than the HD video. Second it is in replay mode which means
30fps and added effects like motion-blur, depth of field and higher detail and better lightning (real ingame play vs replay mode).
Third there is only one car to render and the track detail is below average, prebaked shadows and more (meaning more rendering power to render the car). Considering that in DIRT you can race with 8 cars all visible on tracks with far more detail than GT:HDs track.
So think about it, GT:HD has only one car (youre car) meaning no AI calculations, physics for only one car, only one car rendered. So of course the car could look better.
Really it is much like a techdemo, where a high detail object is rendered, but in a game you could not have multiple of those high detail objects and would have to scale down on the details!