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The calories burned are BMR calories. Your fitbit also calculates calories burned for your BMR. You burn calories just by being alive! Even Sleeping. Fitbit includes these burned calories in it's calorie calculations, so even if you did not wear the fitbit, you'll still burn these calories.
Read this https://help.fitbit.com/articles/en_US/Help_article/1141/
See this thread as well by @ColinM39
Wendy | CA | Moto G6 Android
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