Discrepancy between sleeping HR graph and statistics

[BenSt]

I will attach 3 screen shots sleeping heart rate. Text follows the photos. 

 

 

 in each screen shot of heart rate while sleeping the numerical percentages don’t match the graph. In the third you can see it barely went below resting heart rate yet it claimed that 20% of the night it was below. 
why the discrepancy?  

Moderator Edit: Clarified subject and formatting

[SilviaFitbit]

@BenSt Hi there. Thanks for the screenshots attached and the details mentioned. 

When your heart beats, your capillaries expand and contract based on blood volume changes. To determine your heart rate, the optical heart-rate sensor in your Fitbit device flashes its green LEDs many times per second and uses light-sensitive photodiodes to detect these volume changes in the capillaries above your wrist. Then your device calculates how many times your heart beats per minute (bpm). The optical heart-rate sensor detects a range of 30-220 bpm.

We use green LEDS because they maximize the signal detected from the capillaries near the surface of the skin. The optical heart-rate sensor also uses infrared light to determine when the device is on your wrist to improve the accuracy of your heart-rate data. Learn more about it by checking this article. 

Let me know if your heart rate zones were entered manual or if they haven't been changed.

Hope this helps. 

[BenSt]

Thank you for your explanation of how it measures. It was interesting
to understand.

The only thing was I might not have phrased my question properly. The
graphs I sent show the heartbeat rate while asleep above the average
resting heartbeat for a very large amount of the night, yet the numbers
that were written show me being below the average resting heart for a
majority of the night. This happened those three times making me
hesitant about the accuracy of the measurements or the graphic presentation
of the numbers. Most of the other days I have used it (about 6 months so
far) have been more or less OK.
I just wondered why it would show almost the entire night above the line
yet the numbers showed a much larger percentage below the line. Can
you see the numbers in the photos and compare them with the graphs?
Thanks.

[SilviaFitbit]

@BenSt Thank you for the detailed information provided. I've shared this with my team and they will further investigate it, as soon as I have more information, I'll let you know. 

Have a good weekend.