What i'm saying though is the 225's on my boleros have a bigger difference.
the RS4 has 10" wide tyres on 8.5" rims.
I'm running 8.8" tyres on 7" rims.
The sidewalls are perfectly parallel though.
The measurement of rim width is taken across the insides of the wheel mounting flange, ie the bit the bead seats onto, not the external face of the wheel, so its competely normal to expect quite a difference between the two.
You also need to remember the sidewalls are designed to support the tread, if you've stretched them onto a wide rim (not saying you have, but just talking about the theory) then the sidewalls are pulled taught, and cant properly control the treads movement. On a narrower rim (ie one the correct size) the tread will move around, but it will move around in a way that the tyre manufacturer has designed it to move, in order to maximise traction under load etc.