To that point: I had the opportunity to serve as a member of the medical staff for the '96 Olympics in Atlanta and one of the venues I covered was men's volleyball. I got to know the Chinese "sports therapist" as he spoke english very well.
According to him, and this information is over 20 years old so take it for what it's worth, for many years Chinese girls could be sent to the national sports federations in exchange for a voucher that the child would not count toward the "one child policy". Peasants and farmers were allowed a second child if the first was a girl, but were out of luck if that 2nd child was also female, unless they "gave" the child to an approved national organization.
Once at the foundation, as pointed out by decentshot, kids were tested heavily mentally, physically, and psychologically to determine sports in which they might excel. They were then put in intense training regiments along with regular school work. As they reached puberty, they were reevaluated to see if physical stature matched up with the sport they were in...for example a great gymnast that gets too tall may be moved over into the swimming and diving program.
Now, I have no idea if pool was ever one of the national sports since it isn't in the olympics, and the "one child policy" ended in 2015, so who knows how it works now. But it could possibly contribute to the seemingly huge number of Chinese women who excel at certain sports...TIFWIW