The calorie burn you see is not just activity specific--it is a combination of calories burned related to the tracker's recorded data, any activities you log manually, and your estimated basal metabolic rate (BMR). If your tracker measures heart rate, the calorie burn estimate also takes heart rate into account.
Your BMR is the rate at which you burn calories at rest just to maintain vital body functions like breathing, heartbeat and brain activity. Your BMR usually accounts for at least half of the calories you burn in a day and is estimated based on the physical data you entered when you set up your account: gender, age, height and weight. Your profile and tracker use that data to configure your daily BMR. Note that you can edit your account settings to change any of your physical data.
Your tracker's calorie count will reset each night at midnight and begin counting immediately thereafter. BMR is the reason your tracker starts the day with calories already burned--even if you haven't been active yet, you've still burned calories just by being alive.
March 16, 2015