Is probably ~8lbs water. I'd be amazed if someone could gain 10lbs of fat in a week. That clocks in at about 50k total calories?
When I'm running low carb (for whatever reason) and have a carb refeed, I routinely gain 8-10 pounds in a 24 hour period.
Same for people making a weight for powerlifting/boxing/wrestling. They manipulate their water/salt, getting all the glycogen out of their system before reintroducing them to "pump" them back up etc etc