Those who eat less than they should are shocked and upset when they actually gain weight. If you go back to Pounds and Inches, you will see that Dr. Simeons comments on people that eat less than they should on P3 and how it results in a weight gain. When they go back to the basics, counting calories, watching protein and fat amounts, etc., they lose and are able to stay in the maintaining range.
One of the reasons we suggest that people calculate the calories, especially in the beginning of P3, is to help them understand the amount of calories as well as their protein and fat intake that they need in order to maintain their weight. If you eat too little you will actually gain weight and no body wants to gain! If you are in Phase 3 and you are frustrated with a weight gain, go back to the basics! Use fitday.com and calculate your calories, count your protein and fat percentages and you will be back on track!