Ionic is tracking less distance than other apps

[Woody1953]

My FitBit Ionic says that I walked 2.87 miles.  RunKeeper, MapMyWalk and the odometer on my truck say 2.36.  3 out of 4 tell me that my Ionic is the one that is incorrect.  It is consistently higher than the other apps.  Why the discrepancy?

Moderator Edit: Clarified subject

[SilviaFitbit]

@Woody1953 Welcome to the Fitbit Community. Thanks for the details mentioned. 

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?

Feel free to check out this article about the accuracy of Fitbit devices. 

Hope this helps.