Ahhh, you found the trap I was setting for albaby!

Let's open it up. I'd like a scientific explanation behind this gender identity

It's not really much of a trap. You've noted that there are dozens and dozens of different terms for genders, and that they vary from culture to culture. Which isn't really much more remarkable than the fact that there are dozens and dozens of different terms that are used for, say, "mother" in different cultures and countries.

Or that in different societies, they will use ascribe slightly different characteristics to whatever their local terminology for a trans or non-gendered person is. Again, not very remarkable. It's like making fun of the fact that there are so many different variations on what people consider the role of an "aunt" or a "uncle" to be in different cultures. That's because what it means to be an "aunt" is both biological (sister of a parent) and cultural (the role that such a person plays in familiar relationships).

Because there are wide variations in language and culture between different societies, those societies will use different terms and differentiations in discussing gender. That's a pretty toothless trap. You can replicate the same result just looking at the genders that you acknowledge are real - different societies will have different terms for "male" and "female," and divergent conceptions of what it means to be male and female that will have subtle differences from culture to culture. Or any other psychological phenomena - go to any country and find out what the local colloquial term is for schizophrenia, and the different characteristics that attributed to that condition. That doesn't mean there's no scientific explanation that (or how) schizophrenia exists.

There is a well-established scientific basis for concluding that gender dysphoria exists. Because gender is also a cultural phenomenon, you'll get lots of variations of how societies describe gender dysphoric individuals. They'll use different terms, and ascribe different traits to them.