How accurate is a FitBit at estimating calorie burn?

Photo(1)Last month, I did a little experiment with my FitBit Flex to see if being in a 250-calorie-per-day deficit actually does lead to half a pound per week of fat loss (because there are 3,500 calories in a pound of fat, so 7 days x -250 calories less per day = -1,750 less per week).

I tracked my caloric intake closely using MyFitnessPal in conjunction with my FitBit, and added in any workouts using calories tracked from my heart rate monitor (I burn much less than what my FitBit estimates, so I manually enter in these calories burnt and it recalculates), trying to stay within the range of a 1,750-calorie-per-week deficit. Here are the results: