My theory? We simply don't soak up as much of the culture around us as most people do. If you don't receive social signals too well, you don't get as much of the pressure to be either male or female, to fit yourself into a gender stereotype. So, if you're naturally androgynous, you're likely enough to just stay that way instead of adopting whatever identity seems to fit best. After all, gender dysphoria isn't universal even among people who don't fit into the usual gender categories.

Take me for example: I'm androgynous, but I don't really mind when people assume I'm female, because I couldn't care less one way or the other. I have the usual set of female equipment, but my self-image doesn't really have a gender. If I'd been pushed into gender categories as a child, maybe I never would've realized just how androgynous I am and just kind of assumed that I was female because I'm biologically female and I don't really have that much of a problem with it. If I weren't autistic, maybe I'd never have realized, maybe I'd never have felt free to get a buzz cut and study engineering; maybe I'd have assumed that my love of cats and crochet meant I was a proper girl. But those things are not aspects of my gender; they're aspects of me.

I think that when neurotypicals identify as non-binary gender of some sort, they must've started out with such a strong identity that no amount of pushing could have changed it. For us, there's not so much identification with cultural norms, so we're freer to identify as something other than cis male or cis female.

Still, once you get to the point that it feels wrong to say you're male or female when you know you're not, it's basically the same experience no matter what your neurology is like. By the time you're old enough to know whether you're male, female, neither, or both, your identity is probably pretty firm, and even if you suddenly got all the social signals loud and clear you probably still couldn't change it.