The male/female, man/woman thing is not as easy to define as it must seem if you take it only from pure biology.
As a trans woman am I different from someone born a woman? Of course. Hard to argue otherwise. Science has improved the ability to transition greatly but it is still only a partial physical transition at best.
Am I man, though? Absolutely not in any way I would consider that valid or that anyone who has known me when I was a trans child 50 years ago and the 45 years post transition - all my adult life - living as a woman.
Being a woman is about more than biology.
Oh, and being trans (in my case anyway) is nothing to do with liking punk or pretty dresses and make up. It is about an internal sense of self. Pretty amorphous and hard to describe so appearing to be trite I expect. But the reality nonetheless.
I cannot explain its cause any more than you can - only that it was real and increasingly evident from pre school onward and clear by primary school. This was in an age (the late 50s) when there was no social media and no obsession with gender and no such thing as being trans - the first doctors I saw had no clue what was going on with me and I got shunted from psychiatrist to psychologist to endocrinologist and so on.
Also I was never a cross dresser. The first time I did this, if that's how you phrase it, was the day I transitioned full time 45 years ago and, from my perspective, finally became myself.
I understand why that is hard to understand. It is for me too and I have lived with it forever. But it is what it is and one day I expect science will figure out why.
They haven't yet, though. Or this would not be still causing such terrible trauma for a few kids every year even now.
And it IS a few. Not huge numbers.