Rehydration and renourishment.
Many fighters will cut up to 5lbs by abstaining from food for 24hrs before a weigh in.
Clearing the intestines provides some substantial weight loss.
Emergency cuts can involve dehydration where some fighters can cut 10-15lbs.

But rehydrating and renourishing are an entirely different story.
Thus why some fighters have a remarkable different physique in the fight than the day previous.