Fitbit keeps track of your BMR (basal metabolic rate) throughout the day in addition to calories burned from the steps you take. The total calories that it reports at the end of the day includes both of these things. This number is based on your weight, height, gender and age that you entered in your profile on the Fitbit website.
It also won't be accurate if you just put it on now and expect to see zero, will it? What about what you've already burned today?
I would assume (I don't have one) that this is the fitbit's approximation of what you've burned today already based on your height and weight, to which it will slowly add what it measures for what you burn from after you put it on.
If it says zero when you put it on, it's pretty useless...
Fitness Minutes: (28,611)
2/3/13 7:41 A
I just put my zip on this morning, and it said I had already burned 403 calories? How is that possible when I didn't use it overnight? It won't be accurate if that keeps happening and I might have to return it.