Strava is fun to fool around with, but you can't take it too seriously. I would say that the top places on segments that are popular and reasonable (as opposed to 18 grade downhill for example, or with six stoplights, including a MUP segment, etc) are representative of pretty fast riders. As for the rest of the rankings I wouldn't draw any strong conclusions.

Regarding the averages, you exclude the cyclists who are just riding around and don't care about speed and probably don't use Strava or perhaps don't know it exists. I say that pretty strongly because I'll see I guess ten times as many cyclists as show up on a daily segment. Probably more than that, and they're mostly riding at speeds which would be near or below the segment last place. Excluding them though, I'd wager that the Strava average is about the same as the roadie average.