Distance on treadmill is calculated as number of steps times stride length. If you were holding a hand rail, steps often are not counted as the wrist sees no movement. Fewer steps means lower calculated distance. If you put the tracker in your pocket or attached to clothing somehow, you will probably get good step count but lose heart rate data for that time.
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.
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