1. Carbs don't sit in your body dry, they are in solution. 2.7g of water per 1g of carbohydrate (as stored glycogen). Lowering your glycogen stores drops a lot of weight fast.
2. Greatly reduced sodium intake reduces how much water you are holding, this can be a LOT of weight.
3. Food in the intestines: 4-6 meals moving through at any given time is a LOT of weight.
4. Rapid fat loss in first week or two of diet, this can be double normal or more.
Combine all of these and, depending on the person, size, diet, bodyfat, etc, you can drop 10-20 lbs easy.