For the TL;DW crowd he basically says that at the beginning of the year the Pats were trying to become more explosive because Bill knows we need that to compete with the class of the AFC and with that comes more mistakes. He doesn't absolve Mac for the mistakes that he made during that time, but he does point out they haven't tried anything like that with Zappe, and that's a lot of the reason why Zappe looks better.
It's degree of difficulty, and this is what I have been thinking since Zappe started playing. OF COURSE Zappe hasn't made the same mistakes as Mac because he hasn't been asked to do the things Mac was early in the season. Zappe has basically been asked to do what Mac did last year.