I'd be interested to know if Strava is only using the GPS or if it's using the accelerometers too. If it's just the GPS, it doesn't really know how fast it's going, only where it was each time it solved the equations for the satellite signals. That's never been perfect (ask a geocacher) and deriving the speed, incline, power etc. is only as good as the input data. A couple of wonky points, due to natural error, a swerve, a shift, riding under a tree, etc can really throw it off. Averages should be pretty good, but the peak should probably be ignored.