Due to an excessive runout (side-to-side wobble) of the brake rotor, the thickness of the brake rotor is uneven

If the runout is more than 0.04 mm {0.002 in} at the position 10 mm {0.39 in} from the brake rotor edge, uneven wear occurs on the brake rotor because the pad contacts the plate unevenly.
If the runout is less than 0.04 mm {0.002 in} , uneven wear does not occur.