How Strava calculates maximum speed:
Strava calculates your max speed in a such a way that a GPS error can cause the value to be greater than expected. To calculate max speed, Strava looks for the fastest speed recorded between any two GPS coordinates on your activity. If, for example, one GPS coordinate is recorded + or - 50ft from where you actually are, and the next GPS coordinate is also recorded + or - 50ft from where you actually are in the other direction, Strava will think you are moving faster than you really are because the distance between the two GPS coordinates isn't accurate. On the Analysis graph there is a little bit of smoothing because we do not plot every data point but instead every 3 or so to make a readable graph. If you look at the analysis graph, you usually won't find the same max speed as is reported in the activity stats because the graph is smoothed.