Could someone please explain this to me

Ok, Tuesday is my weigh in day.

Monday I had a horrible day, I didn't manage to get any exercise in and I barely ate anything. (around 900 calories)

Tuesday, I weighed in, 2lbs down from last week, not bad. But then I ate everything in sight. (around 1800 calories) Seriously I just kept munching and munching and munching.

So I weigh in just a minute ago to see what kind of damage I'd done and I'm down another 2.5 pounds... this after having breakfast and a metric butt ton of water.

How does that happen?