I would add one additional reason to Porthardy's excellent reply. I just learned from my coach that when we are on this diet we are eating such low calories that (in her own words) we are 'tricking our body" such that it doesn't think we are starving. If we suddenly throw a ton of extra calories in a particular day, our body does not treat them like a normal day, instead the body "saves" the calories and stores them, because again, in Phase 1, we are one step from our body thinking we are starving. (which doesnt explain why you still lost weight

).
So, bottom line, if we were to phase off the diet, slowly following IP guidelines, we could easily eat a much higher quantity of calories w/o gaining anything (assuming we are not eating a crazy amount). The problem lies with the fact we are eating them in Phase 1 & they get "stored." With cheating, for some at least, we get in a cycle of losing the same weight over and over. When really, if we are not going to be 100% it is better to phase off as these extra calories will not have the same impact if we are out of phase 1.
This is a new angle of the diet that I had not previously understood. Please correct me if my understanding is incorrect. Thanks everyone