Those could be your BMR (basal metabolic rate) calories that you are always burning just by being alive. They are always credited, even when not wearing the tracker, and are put into history as well.
If you did a factory reset to zero out steps, it still figures you have been alive all day, thus burning those BMR calories.
See How does Fitbit estimate how many calories I've burned?
Best AnswerYa but theres no way that I burned 10,000 calories yesterday or I'd be -100 lbs. My fot bit use to read about 300 calories when I wake up. This morning not even wearing my Fitbit it was at 1600. It was working fine and accurate up until yesterday when the calories just sky rocketed
Best Answer
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It's worth checking that your profile settings haven't accidentally changed at https://www.fitbit.com/user/profile/edit Your age, weight, height etc all directly affect the BMR calculations.