Strava and Garmin.

[nogoodnamesleft]

If GPS is doing the measuring I wonder if the nature of the route might affect the outcome. GPS devices record a route by a series of points along your route; so a very twisty route and recording lines between points will cut the corners. Plus different devices will probably be recording points at different intervals and at different times.

On one of my cycles I run a Garmin using an ANT wheel sensor and a Sigma GPS recording using GPS. On a ride mainly long straight sections the Sigma using GPS will record a slightly longer distance than Garmin with wheel sensor. But on very twisty route Garmin/wheel sensor records slightly longer distance than Sigma using GPS.

 

[Spiderweb]

I always use both Strava and my Garmin Edge 1000 just in case one fails. The Garmin always shows slightly less distance and a slightly better average speed.