There are a few factors, even for identical courses: speed sensors, differences in how often GPS data is collected, as well as riding techniques like whether or not one makes abrupt stops.
Red lights are generally gonna kill average speed, so anyone who starts recording in town (or rides through town to get to the ride) is going to have a lower average speed, and IME, coasting to a light to avoid unclipping kills it even more. Check some local Strava weekly leaderboards: guys and gals with the highest average speeds are driving somewhere and letting it rip, but not doing any commuting or around-town riding.