You do lose a lot of water weight at the beginning of a diet, it's true. The postop period is almost a "start over". Everything is thrown into chaos with the shock if the surgery, IV fluids, lack of calories, and many people can't make their fluid goals for the first week or two, so there is dehydration.
The body cycles through fluid unbelievably quickly. So when a person puts on 3, 4, 5 pounds overnight, it's just fluid. Maybe they were dehydrated, and now it's replenished, or perhaps they're storing extra because of stress (like Drop said).. there are lots of reasons for it.. just part of the process...