Answer:
After losing the weight, he obviously did not stay in a calorie balance, as calorie imbalances are the cause of weight loss/gain, but rather he created a positive calorie balance, meaning he took in more calories than he burned. Belinda did the same, as she gained 15lb back, but her calorie intake to output ratio was not as great as Alan's, meaning she gained less weight than him in those three years.
Explanation:
Keep in mind 3500 kcal = 1lb, and you cannot lose fat cells. They just shrink, making people who have once been overweight but lost the extra weight more likely to gain it back.