Also in the starting strength book, Rippetoe claims Zachary put on 55lb in 11 weeks of which 60% was muscle. This is 33lb of muscle, or rather nicely, 3lb a week, which is roughly 12lb a month.
This "student" is clearly the shining example by Mark Rippetoe and he only managed to gain 1lb a month more than what you stated is the expected of 11lb a month with adequate nutrition and training. Other professionals have claimed that what Rippetoe achieved was turning a skinny kid into a lard arse with bad form when lifting and also that he was innacurate with body fat measurements.
Don't get me wrong, his transformation is fantastic and lifts are very impressive, but even if the case of Zach is 100% accurate, it is down to amazing genetics that he can achieve this and it is not a feat that every normal guy can expect to achieve.