I dunno, that's how I lose and gain weight. One day I will weigh myself and be the normal weight, and then the next day I wake up and weigh myself and I'm a pound or two lighter. It is usually a weekend day that that happens, but it usually sticks. I think that when I'm tired I retain more water and then getting that extra sleep helps me release it (could have something to do with eating more carbs and such when I'm tired - I've never figured out why this happens) but that's how it goes for me. Unfortunately, the opposite also happens, which is sort of how I got to where I am. When the scale goes up, you just hope it's water, but usually when it goes up, that weight sticks around
I just think every one is different and it doesn't matter how much you say that can't happen and that's not possible, some people just don't fall in that norm. I've been in this body 29 years and I still can't figure out everything that makes it tick, but what I have figured out is from consistent tracking and trending. You just have to keep your eyes on your page and do what helps you. And, what I have learned from this book is, to be really sure you can only change one thing at a time to figure out what's going on. It frustrates the hell out of me, but I just want to know what works for me, for once, and tracking and checking is the only way to sort it out.