Player Development

When I'm looking for an assistant coach to handle practices, I've always gone with coaches that have a high Player Development rating, regardless of what the other ratings are, and it seems like every year, the players either stay the "same" (potential goes down) or their skills actually regress. However, if the assistant coach has high offense/defense ratings but low Player Development rating, I don't understand why they would be better for... player development. Any help?