it could be many reasons, i think we all have our own theories
personally i think that sometimes its just your body adapting to the weight loss ......... i had a 0.5lb gain the other week for no real reason and the week before that i'd "only" has a 1lb loss .... (despite sticking to the diet both weeks) but i know i've been loosing the inches the last few weeks (as my trousers keep falling down ) so i think my body was holding onto the food while it was changing shape as last week it was a 2lb loss again on the scales (which is about the average loss for me to loose a week)
just keep up the good work and don't let a gain on the scales get you down