In my film distribution class, it was revealed that while most films are marketted to men in the west (because it is presumed that women will be going with men, & the man will ultimately choose the film), in Japan, films are marketted to young, single women, who, with their office jobs, have disposable income & time after work to go together with other office gals to the movies while the men at the office hang out in social places more "appropriate" for men-only.
Perhaps the same is true for single Japanese women in the diving world; perhaps they move independently to persue sport recreations without waiting for a man to sign up with them. We can't necessarily expect to find women who dive for peals in statistics because they may not be certified, so there may be lots more than we think!