Yes. The only way the carbs are stored in the human body is in the form of glycogen, in liver and skeletal muscle. There's about 1 kg of it in total, maybe a bit more if you're a trained athlete. And glycogen binds about, maybe, up to 3 times as much water by weight. So, altogether, about 4 kg.
After switching to keto, glycogen is used up gradually, due to no carbs entering the body. And the water is also lost. Therefore, the "signing bonus" of the keto diet - a significant weight loss in the beginning, but due to water lost, not fat burned.
BUT. That only accounts for about 4 kilos, maybe just a bit more. You can't really blame water for a gain of 20 pounds.
No one ever said it could account for 20 lbs. Merely that you should expect weight loss when you're cutting carbs and weight gain when you reintroduce carbs.
