I burn more than 400 calories just by sleeping.
Just to qualify, I don't believe in using such devices - way better to just be brutally honest with yourself whether you are getting enough exercise, and make it a way of life than to convince yourself to meet some minimum standards set by someone else.
But from my limited understanding, Fitbit does include Basal Metabolic Rate (BMR) in its measurements: see
here.
And you do burn calories even when sleeping, else someone who is bedridden would never need to eat.
So anyways, based on a random BMR calculator
here, a sedentary (read: no exercise) man of age 50 at 170 cm and 65 kg is estimated to burn about 1473 calories even doing nothing. This will of course vary depending on your physical makeup (% body fat, % weight as muscle, blah blah blah).
I'm not sure about your steps and calories though - gotta actually try one to know if this BMR thing featured as well, and I'm not interested!
[Back into hibernation...]