@MorrisZapp
I don't know, and neither does your fitbit. And if you did burn 500, did it come from your pre-workout banana, from your body fat, or from calories that would have otherwise gone into keeping your skin clear and your tail bushy? Chances are it's a mix of everything, so '3500 kcal = 1lb of fat loss' is a bit nuts. That's how many kcals are in average fat cells, which we don't have (any more than we are each 'average height'), and that's when it's burned in a bomb calorimeter, usually in a lab under scientific conditions. Our bodies aren't bomb calorimeters!
Your best bet with calorie counting is to work out how the numbers correspond to you. So if you calculate a 400kcal deficit, and you lose weight too fast, try a 300kcal deficit instead. If you don't lose it fast enough, try a bigger deficit.
It's all rough estimates based on averages, and it gives us a general idea, but really, I might burn 300kcal from doing something, and take it mostly from my banana, and you might burn 600kcal doing precisely the same thing, and take it mainly from your body fat.
Your body is your scientific experiment, and it represents to you what you've been doing consistently for a long while. If you want it thinner, give it less fuel, and burn more fuel by moving about. If you want it fatter, eat more fuel and burn less. That all stands true, but there are so many variables re encouraging your body to burn more fat/maintain healthy homeostasis that the numbers aren't really to be relied on.