Although I agree about eat less, move more, if we don't eat enough, and exercise too much it is perceived as a strain on the body and sometimes weight stays put as a defensive mechanism. I wish it was a simple in/out equation because if that was the case I would have dropped the nagging 5 extra lbs by now! LOL! For many, thyroid issues come into play (possibly even sub-clinical).
I don't have weight to lose, per se. I was always very thin, so by most people's standards, they would think I was crazy to want to lose any. It has just been an odd thing to actually have to watch what I eat and to have gained a bit in the past 4-6 years. But I have read of so many women watching what they eat and getting enough exercise and yet still struggling with weight issues.
I think it really ends up being just a very individual physiological experience - no two women are alike and no two ways of dieting or exercising work the same for everyone.