I bought a radar gun a few years ago and left it at my local club for the players to try out and people were consistently getting results lower than they expected.
Of course nearly everyone thought they could serve at least 100 mph, but the reality was they were serving in the 70s to, at most, the 80s. Even my doubles partner who has a pretty decent serve was only getting in the 90s.
The number of folks who managed to achieve three digits were countable on one hand, but they were the best players we had around at the time (except me, for some reason I managed to do it too).
Moral of the story, most of us overestimate our abilities, it's human nature.
My latest video: http://www.youtube.com/watch?v=bjTW-BPHqIE