There's a large difference actually. Rear lights, carbon fiber detail and model, engine is added, interior is added etc.
Shame how crappy the pics are but these changes will make much difference, also shadows and AA on top adds even more to the impression. In the AUDI top down there looks to be more changes in the poly count on the tailgate but its just a crappy blurry video so not going to say for sure.
Nothing wrong with it though, Forza does this.
When I want to photograph a car I want the best model possible, fully tessellated as well. I'm glad that Forza swaps in the models.