Socially, there are more than two genders, however, genetically there are two gender types, yet there are variations such as XYY syndrome. So Socially you can identify differently or act differently, at a social level you can use a label/identity/role as a persona, however, the fact do not change. I don't think there is a movement to say that 'chromosomes change', or anything of this kind, a rare few will have chromosomal conditions that are non-binary, the previously noted XYY condition is a common example. Other conditions that may be a factor vary, such as stable chemical differences, thus a lower amount of gender specific chemicals being produced by the body.
And, regardless of all of this, there are those who are non-trans and do not have gender dysphoria as much as general identity issues, so confusion or preference allows some to confuse identity issues with gender identity issues. As a result, we have people claiming trans and fluid gender types based on personal mental-emotional issues while they're young and being exposed to new ideas in university. It's a bit like students who get politicised, in fact, it is the same in many ways and politically social issues are pushed forward by those who fail to define what is social and what is scientifically true.
There are those who don't trust science and will reject it for social ideological views, they, when they appear, are the troubling phenomenon that is growing out of social justice politics. And, they are the true core of the trouble with social issue activists.
My video on this subject: