On a racing car, the discs are mounted to the bells using a bobbin arrangement that allows for radial and linear disc movement: radial for expansion and contraction due to heat, and linear for reducing pad knock-off. Pad knock-off is caused by flexure in the hub and bearing assembly when cornering: the disc pushes against the pads and moves them away from the disc surface. When you next apply the brakes, the initial pedal travel is taken up moving the pads back to the disc surface again. Racing calipers further reduce this by having springs fitted behind the pistons. A fully floating disc assembly is quite noisy as the disc rattles when the brakes are applied. This is not the sort of thing you want for your road car, so the OE disc manufacturer fits anti-rattle spring clips to the bobbins. This effectively reduces or possibly even stops any true float that the discs may have as the spring clips are quite stiff. After a while road dirt and corrosion build up in the bobbins further reducing any float. And finally to the crux of this long winded waffle. When you have cornered hard and flexure has caused the disc to push against the pads, the disc moves in the bobbins but doesn't naturally return to a central position due to all the road dirt and corrosion. We know that the 8-piston caliper has really only one problem and that is also with corrosion. This corrosion occurs under the pad wear plates causing the pads to jam. With the poor design of the disc for the road and too tightly fitting pads, disc runout is the end result.
Of course, the cast iron disc can also warp, and uneven pad material deposit can cause harshness to the brakes, but the RS4 front disc is quite unique in its (poor) design.
Lastly, I attach a picture of a disc I skimmed recently for an RS4 owner. The discs had only covered 1000 miles or so, and he was complaining of awful brake judder. You can see quite clearly the amount of runout from the uneven cut. (Look at the right hand side of the disc: this is where the cutter is cutting first. After three passes of the cutter, the discs were spot on and the car's owner has been very happy since.
