Inspire HR or App? Not calculating distance properly from steps taken

[netgal]

I have an Inspire HR that I use in conjunction with an app on my Iphone.  For some reason the distance is always off - the calculations aren't even right.  I use the "standard" stride length of 65.3 cm - one day I had 6236 steps and it calculated distance as 1.51 km.  Next day was 6897 and it calculated 4.51 km (which makes a little more sense).  Yesterday I had 5.16km from 8052 steps, but today it's saying 4.51km from 8576 steps.  

I've tried logging off the app, restarting the phone, resetting the stride length.  Removing the device and readding it.  Rebooting the Inspire.  The Phone GPS option is turned off.  It doesn't make any sense.  Anyone have this problem and know how to fix it?

[SilviaFitbit]

@netgal Welcome to the Fitbit Community. Thanks for the troubleshooting performed prior to posting. 

I appreciate the detailed information shared. Fitbit devices calculate distance by multiplying steps for stride length. 

When you track an activity with GPS, your device calculates your distance using GPS data rather than steps. If you begin moving before you get a GPS signal, your device calculates distance using your steps until GPS connects.

To update your stride length, track a run with GPS or measure your stride length manually. For more information, see Can I change my stride length in my Fitbit account?