From lifting and studying Body Building to use their studies to help reform my heath.
The initial rapid weight loss is from burning glycogen stores, not Body fat. Glycogen really likes water, so with the glycogen you retain a lot of water. The ratio they would cite was 6:1 water to glycogen.
Initial calorie deficit burns mostly the stored glycogen, releasing a LOT of water. Scale goes down fast. Then real fat loss occurs which is much more inline with 3600 calories per pound of fat...
This turns out to be a GREAT thing in disguise. Yes many get disheartened after the initial dramatic results tail off. But once you've reduced your glycogen storage it helps somewhat reduce blood sugar spikes... Not enough to allow you to splurge, but enough to further dampen spikes from healthy eating. Insulin spikes are very effective at adding to your fat stores.