People who have advantages are given opportunities to further their advantages.
People who are successful in an endeavor, even those only marginally more successful than their peers, are often given the opportunity to practice that endeavor. This additional practice over their slightly less successful peers tends to widen the gap between their levels of success. Because you’re a 6 out of a 10, you are given opportunities (traveling sports teams pick you up) to develop into a 7, 8, 9, or 10. This phenomenon is what Malcolm Gladwell attributes a certain distribution curve of birth months in professional sports.
Hockey Example
In Hockey where your birth year determines what leagues you play in, there’s a large disproportionality skewed towards January, February, & March birthdays. More than double the number of players born in the first quarter of the year than those born in the last quarter of the year. The theory is that at young ages, when a matter of less than a year can actually make a difference in the size and skill levels of players this makes the January boys stand out from the December boys on average. The January birthday kids then get selected for special teams in special leagues getting more opportunities to practice because they’re bigger than the younger players. Then, because they’re playing more, they actually get better than those boys who happened to be born later in the year.