What insights do Barnsley and his wife have about the professional hockey players from the Ontario Junior Hockey League?

From the text:

Barnsley went home that night and looked up the birth dates of as many professional hockey players as he could find. He saw the same pattern. Barnsley, his wife, and a colleague, A. H. Thompson, then gathered statistics on every player in the Ontario Junior Hockey League. The story was the same. More players were born in January than in any other month, and by an overwhelming margin. The second most frequent birth month February. The third March.

He looked at the all-star teams of eleven-year-olds and thirteen-year olds, the young players selected for elite traveling squads. Same story. He looked at the composition of the National Hockey League. Same story. The more he looked, the more Barnsley came to believe that what he was seeing was not a chance occurrence but an iron law of Canadian hockey: in any elite group of hockey playersthe very best of the best40 percent of the players will have
been born between January and March, 30 percent between April and June, 20 percent between July and September, and 10 percent between October and December.