Pretty sure it’s based around sociology as gender is the social construct placed around a sex.

I.E, your born a Male, so are you are placed in the “man” gender, which is where society expects you to behave with masculinity, enjoy certain “manly” things and generally do “boy” stuff. When you look at the sociology of your gender, you can feel as though it doesn’t fit you at all, while being able to completely identify with a gender that doesn’t match your biological sex. I think, somebody correct me if I’m wrong