From what I've read on here before, one possible explanation is leptin...somehow, after a long period of dieting this substance in the body is depleted, and eating differently for a little while tricks the body out of starvation mode and replenishes the leptin levels, helping the body stop holding onto fat. Doing a search for "leptin" and "refeeding" should turn up some interesting stuff. I'd do it myself but I have to get back to work on school stuff!