If I remember correctly (read this book years ago) the argument with the hockey players was that a few months of difference between kids makes a big difference

If I remember correctly (read this book years ago) the argument with the hockey players was that a few months of difference between kids makes a big difference with their physical strength and development and a few months of difference between to adults is not that visible. So the kids who are born in the first few months are able to outperform the younger kids who haven't developed as much.