>>76578732
No one really knows the exact biological mechanism, but its been widely observed and documented.

Pic related is one common explanation, though its mostly debunked, your fat cells can't literally fill with water, though water might be retained somewhere else in your body. Truth is we don't know why despite being an a deficit people's weight can plateau for weeks on end only to suddenly catch up to where they were supposed to be over the course of a few days, but it happens.