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...
Deb, in New Zealand