It seems to me that gender (at the extremes: XX v XY) is the byproduct of a mutation (read: variation). In that context, it makes sense that there are degrees of variation (if you will). That is, it's in the best interest of the species to experiment (if you will) with other (genetic) possibles. The fact that people identify as something non-binary is natural, and (imho) expected.

There's likely more variation than we've previously realized due to social constructs.

Another perspective:

If you consider a gender to be the distinct set of behaviors that achieve the highest genetic fitness for their matched biological sex, then there is likely to be much less variation than current political discourse would suggest.

Why would that lead to less variation? What would cause hormones to not (say) bell curve? And hormones are just one piece in the puzzle.

There are, for example, a diversity of hair color, and that changes over a lifetime. Why would DNA's natural "experiments" with regards to gender be any different?