One explanation, and the most likely, is that the Z06 was sandbagged to make the M5 look faster as M5board panders to M5 owner's sensitives. This type of nonsense though leads to M5 owners thinking their cars are stronger than they really are feeding their egos. We have a video below with a stock Z06 crushing a stock M6 from a roll yet supposedly a tuned Z06 somehow struggles with a stock F10 M5? This is one of those times where a video and its description can be very misleading.
F10 M5 and F12/F13 M6 owners, do yourself a favor and watch this video. Because if you try to pick on a C6 Z06, tuned or stock, with your stock M5/M6 you are going to get your ego bruised and deservedly so. That will happen regardless if a site dedicated to M5's tries to portray the M5/M6 as faster than they really are.