If I may venture an interpretation on that...
Maybe sometimes the variable we don't give enough attention to is the players themselves. We get into the mindset of who makes the most difference, club coaches or high school coaches, and sometimes we forget it is the player who also makes a big difference. I think one reason club players perform better than non-club players is not necessarily because of quality of coaching--although I do not discount the effect a highly-skilled club coach can have--but it's at least partially because the club players tend to be the ones who put more of themselves into the sport. They are the ones who care about playing and playing well, enough to go to the extra effort, time, travel and expense to add another soccer season per year to their lives and work to get better; maybe they improve not so much because of who's been around to give it to them, but because of what they've done to earn it.
