Active minutes are calculated by heart rate, not steps. So if you are sitting still but have an elevated heart rate (I think above 100 bpm), those are logged as active minutes. Even a brief walk can be logged as active minutes, especially if you are walking up stairs or carrying something. Fitbit shows you active minutes so that you can see how many minutes each day you spend being active, even if that activity is not specifically intentional exercise. I also work in an office and receive active minutes for short walks.
If you are receiving active minutes when you are truly being completely inactive, your Fitbit is likely improperly reading your heart rate.
