I know, it's not consistent. The book explains it a little better, how a build-up of the "bad" foods triggers an episode. Brian can eat anything right after he has a migraine, because he never gets one he following day.
AND he gets incredibly depressed, get's an "eff it" attitude and eats whatever the heck he wants. (Not sure if your daughter gets depressed, but the book also describes the seratonin dump)