... You're eating leftovers.

 Bonus lack of calories for not using a plate!

This rule is fairly straightforward. Calories that have already been cooked or prepared in some sort of meal have been "activated," and are therefore on their way to expiration and decline. After one day, the calories in this meal have expired and suffer from complete loss of potency. Refrigeration speeds this process, making your cold leftover pizza practically calorie free!