I heard a theory one time and I don't know if it is really true or not that teams in the AL are quicker to change due to having to find interesting ways to keep up with the Yankees and Red Sox. So AL hitting coaches might have adapted to the fact that pitchers are now basically robots throwing certain pitches in certian counts for strikes or balls the coaches in the NL might not have caught on since there is more competitive balance.
It may also have to do with what pitch is being thrown first. In the AL they may be throwing get ahead fastballs... while in the NL they might try to work in an offspeed pitch.
8 out of the top 10 teams in terms of fastball % thrown a game are AL teams... including the Top 4.
The top 11 teams in terms of slider % thrown a game are all NL teams.
