That's what I thought, too ... piston appears to be moving smoothly (checked it) ... isn't it odd that BOTH rears would wear the same ... maybe any apportioning issue at the master cylinder?
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg