Being in a caloric deficit does not "necessarily" mean the body is taping into its fat stores. The body can and will turn to muscle tissue when a deficit is present. This is modified and reduced when weight training when deficit dieting, but not completely eliminated.
For most healthy persons, one can eat JUNK (hypothetically), exercise, and as long as the law of energy balance tips to the negative side more often than not, one will lose-----tissue.
For most persons, and largely dependent on several factors, one can cheat on a meal or two, and IF it falls within the caloric limitations (on the deficit side), the body (when things are considered equal), will have no choice but to look for more energy (calorie) else where. The draw back in most cheat meals (not all of them, if a PLAN is in place for these times), is the nutrients can cause some negative effects on the goal being sought after (these nutrients can dependent on what was eaten), hamper recovery and things needed by the body for various other operations.
When deficit dieting, the body and the mind is going to crave certain things the person KNOWS they shouldnt have. What I dont understand, is why persons act on a whim instead of embracing this normal craving and have a carefully selected plan altered to the these individual impulses, instead of acting so suprised and unprepared mentally for them. If one changes eating habits different from the past, the body and mind is going to resist you---because, well, its different than what its used to (and its no different than when training, you try something different, sometimes you get sore, eh?). Therefore, given time you and the body will adapt together if you embrace these cravings the right way.