Fitbit has recently announced some changes to the way they count user’s active minutes.
Before if at any time you picked up your pace Fitbit would count that increase in effort as “Active” minutes, and simply sum up those minutes each day. However, in compliance with suggestions by the American Heart Association, Fitbit will only count active minutes after 10 minutes of continuous motion. This change will not affect your step count.
As a result, you may find that your Fitbit active minutes will be lower for the foreseeable future.
For more details, read the blog post Fitbit Extends Minimum Time Frame for Active Minutes.