What's the difference between Fitbit active minutes and active zone minutes?

Exist can sync your active minutes from Fitbit to your Exist account. You might see something called "active zone minutes" in the Fitbit app, which is a different value to your active minutes. Fitbit's active zone minutes are related to your heart rate during exercise, and the value can be higher than the actual number of minutes you spent being active. Fitbit's active minutes, which sync to Exist, are a count of minutes in the day that you spent above a threshold that Fitbit determines as being "active".

Read more on the Fitbit site.