The initial comments at slashdots focuses on the accuracy of the data and soon shifted to speculation of the causes. In his book Outliers: The Story of Success, Malcom Gladwell writes about the birth month of players in the Ontario Junior Hockey League. More players were born in January than in any other month. Why? Read Chapter 1 of the book.
Want to know the answer now?
The explanation for this is quite simple. It has nothing to do with astrology, nor is there anything magical about the first three months of the year. It's simply that in Canada the eligibility cutoff for age-class hockey is January i. A boy who turns ten on January 2, then, could be playing alongside someone who doesn't turn ten until the end of the year—and at that age, in preadolescence, a twelve-month gap in age represents an enormous difference in
This being Canada, the most hockey-crazed country on earth, coaches start to select players for the traveling "rep" squad—the all-star teams—at the age of nine or ten, and of course they are more likely to view as talented the bigger and more coordinated players, who have had
the benefit of critical extra months of maturity.
And what happens when a player gets chosen for a rep squad? He gets better coaching, and his teammates are better, and he plays fifty or seventy-five games a season instead of twenty games a season like those left behind in the "house" league, and he practices twice as much as, or even three times more than, he would have otherwise. In the beginning, his advantage isn't so much
that he is inherently better but only that he is a little older. But by the age of thirteen or fourteen, with the benefit of better coaching and all that extra practice under his belt,
he really is better, so he's the one more likely to make it to the Major Junior A league, and from there into the big leagues.""
The cause, suggested by the original researcher can be found here.