To add my 2 cents to this...
My son is a 2012 LHP. He's always been pretty effective, he competes and he throws well.
As a dad, I got excited in bull pen sessions with the radar guns showing 85-86 as an incomming freshman. What I've had to deal with is:
1.) Not all radar guns are the same
2.) Jr. throws to location with a batter in the box in game situations and the velocity drops
3.) Jr. doesn't throw to location without a batter in the box in non-game situations and the velocity brings dreams of early draft picks.
4.) Jr. doesn't throw the same way consistently day in and day out.
During this pro-scout fall season, he's gone from sitting at 84-86 in a "fall classic" in front of lots of scouts. To throwing 78-79 against a CC and not 'breaking glass' (despite striking out the 1,2,3 hitters in the first inning). It's very humbling when they start gunning every single pitch Jr. throws. There is no place to hide when the numbers don't seem to be high enough and they start asking, "What's wrong with JR today?"
There are lots of consistency issues with young pitchers. Jr can throw a ball 86 if somebody asks him to. On a really good day it'll be 88, but he can't 'pitch' at 86 or 88. He pitches on his best days at 84-85 and can be effective at 79-82 against quality HS hitters. Like I said, he struck out three straight CC hitters and hit 82 one time with most FB at 79-81.
I've heard there is a "ask a kid how hard he throws and subtract 5 mph rule". I think a lot of the above plays into this.
What does it mean? What velocity are we talking about when we talk about velocity?
1.) Sitting in the 5th inning?
2.) Overthrowing in the bullpen?
3.) Top speed in the game of his life?
4.) Average top speed over 10 starts?
Kids are kids and these numbers will vary.
Pros are gunned on every pitch. Any slight inconsistancy is documented and analyzed. The average HS kid hasn't had that happen just yet and is better for it.