I suspect that records move in spurts, not continuously, when a star athlete develops. Like Gray Umbach or Jimmy Feigen. Both own several. Vollmer was obviously a superstar.
This year the boys got a new 200 free record (1:36). That's noteworthy because so many kids swim that event.
Think about how long Joe Hudepohl's national 100 free record stood. Or Jeff Kostoff's 500 free national high school record from 1983 (4:16) broken just this year by Jack Conger (Texas recruit). That record stood for 30 years!
I also think sometimes elite swimmers choose not to swim for their high school team. As much as I can tell, Simone Manuel, from Fort Bend Austin high, a WC finalist in Barcelona, didn't complete last year in high school. I can't imagine she shouldn't have crushed the field in the 50 and the 100.