It's deffo psychological, though i think it's the 'former fat girl' mentality. When i was at my biggest i'd still obsess over my weight, get really miserable when I weighed myself, go out and eat enough chinese food to feed four people, sometimes with fish n chips for afters, then weigh myself again.
I can 'put on' up to half a stone in one sitting you know? Then the next day when 5lbs of that was gone i'd be all like 'Oh wow i've lost 5lbs eating loads of food how good am I?' when really i'd put ON 2lbs overnight.