I was sitting next to a scout at a game with a pitcher throwing in the low 90's. Suddenly one pitch registered at 98. The scout said, "oh, thats the bat." How does he know? Could be right because it was 5 mph faster than the other readings - but how does he know when a batter swings whether the radar picks up the pitch or bat for a guy swinging a similar speed as the pitcher is throwing?
Also, I would think that whats important is that a given scout is using the same gun day after day, week after week. So if he measures someone at high 80's on Tuesday and then the next guy at low 90's on Wednesday, then he knows the 2nd guy throws a little harder than the first. Doesn't seem to me that it matters within 1 or 2 mph whether or not his gun is EXACTLY right - just the relative recordings from pitch to pitch and game to game.
90% of the guns I saw at events are stalkers. The other 10% with Jugs are dads or college coaches who haven't bought a new gun lately. I've sat behind them both - they're not very different - as others have said, slightly higher on the Jugs on average, but not necessarily on every pitch.
________________
"Pitching is the art of instilling fear," Sandy Koufax.