Just because you eat 1200 calories most days does not mean you are eating 1200/day. Those days off plan can seriously distort those numbers, adding 200-300 more calories to your daily average.

Do you use an on-line calorie tracker? If you don't, I recommend it, and record EVERYTHING--especially "cheat" meals. You don't know how many calories you are eating "on average" until you have a couple weeks to average. What you eat in a given day is such a small snapshot that it is really meaningless all by itself.