Strava does the exact same thing to me.
I sync rides from Polar GPS to Polar Flow and then out to both Strava and Training Peaks. The Polar GPS, Polar Flow and Training Peaks all agree but Strava consistently under reports both distance and average speed.
Saturday I rode 66.2 miles @ 14.4 MPH but Strava says it is two miles less and 0.6 MPH slower. Yesterday I rode 1 hr 58 sec on rollers at 22.2 MPH vs 21.9 MPH according to Strava. Group rides are the same - the Polar GPS head unit, Polar Flow and Training Peaks all agree with the other riders but Strava always says I rode less miles and slower than the group.
Three different Polar GPS' on three different bikes, each with their own sensors. Indoor or outdoors, GPS on or off, auto pause enabled, it doesn't matter. Strava always gets it wrong.
I've even tried stopping the ride during long store stops, starting a new ride after the store stop and then combining the two rides using the
GoToes Utility but it doesn't help.
@
TenSpeedV2, you could get a free Training Peaks account just to test. I'll bet that TP agrees with your GPS and Garmin Connect.
-Tim-