It's SOP to pack on weight after the weigh-in but the weigh-in is the official weight. It's pretty hard to believe a guy could put on 21 pounds but I'm told James Toney added 27 pounds once.
Still, it's not against the rules.
It's pretty unhealthy to cut and regain that kind of weight.
But then again, this is boxing and the no one accuses the participants for being either smart nor health conscious.