The "10% at the most" deviation in the MTF for any random sample is also incomplete. As other posters mentioned, there is an occasional lemon. Of course there is! It's IMPOSSIBLE to claim that ALL lenses will deviate by at most some amount. Most manufacturers use a probability model to tell them the tolerances are within so much for some number of samples. It might be something like you get 1 lemon (MTF's are off by more than 10%) every 1000 units or 10,000 units.