The people I know who consider themselves non-binary feel that way largely because they reject the roles and boxes that come with gender. Their personal truth is that they feel like themselves, and don't want to be pigeon-holed by someone else's idea of who and how they should be along a binary. And, there is indeed a wide range of experiences and expressions of Self that are now gaining at least some acceptance (despite the pushback from some quarters). I do find it interesting and curious in the face of this though how many people, even progressives, still cling to the idea that boys and girls are fundamentally different.