Rob T -- Amen to that statement.My 2016 is fairly fast and has run 60s at a camp, a tryout and prospect. At 14 he ran 7.3 at a D-1 college camp. At 15 at a travel team tryout, he was a 7.5 in tennis shoes (forgot to put his spikes on) in long grass, then did a college prospect camp two months later and ran 7.6.
I believe a lot has to do with human reaction time. A coach with a stop watch telling a kid to go, at a distance they walked off, is never going to be accurate.