Most JBS end at 9, and these are huge feeders to bs. Also, many foreign systems have an easier break at 10th. That’s where many of the sophomore applicants come from.
There are always kids who start at their lps and realize bs would be a better option. And there is also that subset of kids whose families are relocating and who feel that bs is preferable to the school where they are going.
Transfers between bs are quite rare. Not saying it doesn’t happen, but it’s probably the smallest group of incoming students.