Here is an example. This was a gravel ride done Nov 24. I'm pretty sure it has something to do with time as well as distance. Strava always has the "Moving Time" a little longer than Polar or Training Peaks "Duration". Heart rate data is typically accurate though.
Polar Flow
The same ride synchronized from Polar Flow to Training Peaks...
The same ride Synchronized from Polar Flow to Strava...
-Tim-