Does your C280 have traction control? If it does, technically it should be less likely to wear. This is due to the traction controls ability to electronically "lock" the rear diff via rear brakes to simulate a limited slip. This will prevent damaging one wheel spins. When only one wheel is spinning, the spider gears inside will start to spin to compensate. These gears arent designed to rotate fast, just fast enough to compensate for the car to go around corners.
It might be possible that your C280 was "abused" at some time in its life. Power braking will cause very bad damage if only one wheel is spinning, in which if you dont have ASR, it will be one wheel. I wish mercedes outfitted most of there cars with limited slip. Your diff could of been mis adjusted internally right from the factory, thus the flakes. Does your diff have excessive play? Do you here a clunk when you go from reverse to drive and vice versa? That is a sign if may be wore. That and the constant drone at highway speeds.
p.s. the arrow is pointing to the spider gears