It's a coping mechanism. If you've been lying about something your whole adult life, there is a very good chance you actually believe it. What makes fat lies the worst is how simple it would be to disprove them, even by yourself. Couting calories for a month would probably take add like 20 minutes combined to your daily routine (not to mention how much time they would save by not eating after hitting that 1500kcal by like 1pm until they adjusted). But no, this is just more comfortable and easier. :)