binge day and weight loss

Hi, I know I have read people saying that often after eating clean for a while, a day where they be at what they like can result in a loss.

So I know it happens to people, buy WHY does it happen? I had a crazy day yesterday, hot chocolate with whipped cream and a flake, piece of cake, shared a bag of nougat bites, huge piece of cod and chips and onion rings for dinner. No exercise except walking round shops.

This morning I thought I should see the result of this and I'd dropped a pound! I really want to understand why this happens.