Generally, your device/app tracks 2 types of calories. First, there are "passive/inactive" calories that you burn for just existing as a human. Secondly, there are "active" calories that are burned by activity (workouts, walking, taking the stairs, etc). We only pull the "active calories" which are shown on your challenge dashboard.
Many activity tracker apps/dashboards expressly break down the categories of calories for you when you're looking on their app/account (total calories, active calories, etc). Fitbit specifically, however, does not show you your active calorie count on the dashboard anymore, only active minutes. In speaking with them, they stated that the algorithm in how they determine number of calories burned per each active minute is private, but it is based on the individual's personal stats (height, weight, gender, age).
For many other companies, they usually show you your total calories & your active calories on the dashboard/app. When this is so, the active calorie count should match the calorie count on your challenge dashboard.
If you have any further questions, or if you think your calorie count is off, please email firstname.lastname@example.org for further assistance.