Firstly, the mother who wrote this letter has only heard about the food from her son and his friends. She has not spoken to other students from his class and not with other parents. It might be that her son is fond of very healthy foods, including excessive fats and calories. He could have plotted with his friends about the issue to deceive her mother so that she can give him healthy food in the lunchbox. In order to know the reality, she must speak to other students and parents and look what ratio exists between the opposite.
Secondly, it can also be a reason