Well isn't it more obvious?
Pitching, Pitching, Pitching!
When a 12u is basically got a couple of pitches and slight movement of course the batters are going to take full swings and tee off on some pitches.
An 18u batter facing a dominant pitcher with 4+ pitches in her bag and anywhere from 3-6+ inches of break on them most 18u batters are not taking full on swings unless they are ahead in the count and guessing fastball.
This probably has some merit.