This issue has persisted since Forza Motorsport 4. The Audi RS2 B4 in Forza has a low quality and inaccurate car model, typical for cars that debuted on the Xbox 360. About everything with it is wrong to some extent, which sums up to a bad looking model. As a person who sees standard Audi 80 B4s all the time, and they have the same dimensions, I can confirm that the Forza model is inaccurate in this regard.
The whole body is too tall and too narrow, which ruins the proportions and the shapes of all parts.
The headlights are slightly too big and very low quality. The doors are slightly too flat
The rear end is probably the worst part on its own. The rear window heating is upside down and too thick, which was already listed as “Will-Not-Fix” shortly after the game’s launch. The rear lights are opaque. The rear bumper is shapeless.
The colour on both cars is supposed to be the same RS Blue Pearl Effect, but the one on Forza is closer to Nogaro Blue, which wasn’t available on the RS2. I’m yet to check other colours, but it’s very likely that RS Blue is the only plain wrong one.
Overall, the Audi RS 2 in Forza should be remodeled. As is the case with the Impreza GD and the NSX NA1, a remodel would go a long way because it’d allow to easier create other versions of the B4.