Good to see you have joined the Fitbit Community @Joniquest! I'm wondering if you are comparing your BMP in real-time between your tracker and your phone while you are doing the exercise? Or is this information off after you have synced your exercise?
On that note, the tracker sometimes might loose connectivity by brief instances that could explain why your heart rate is off in comparison to your tracker.
Also review your exercise details in the exercise history after synced your treadmill walk, to make sure the average heart rate, is similar to your the information reported by your tracker.
In either case, refer always to your tracker data, as this is the immediate source to view your information in real time. In the app either is in real time and after sync it may take a few more seconds to compile your information and may present some delays.
See you around and let me know if you still have questions.
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