People's "how taboo is your sexuality?" score (a score of what people are into weighted by how taboo the things are), compared to their gender and sexual orientation
There's a small contingent of asexuals that are just fetishists to such a degree that they don't experience sexual attraction outside that fetish.
Not that that's gonna account for all of it. But yeah, asexuals masturbating isn't rare.
Asexual friends of mine have described it as, “Some people have fantasies that they’d never want to actually do (noncon or vore, say) and some people are asexual because ALL sexual thoughts feel that way for them- but they might still have odd/kinky thoughts.”