Just let it go, your not interested therefore the ' why's' don't matter.
There could be any number of reasons she is the way she is and you will never know.

Imho, your starting to sound a little obsessed, yourself.
"It doesn't matter"

Why the need to put her neatly in a box. And give her a label.?

What you should be asking yourself is not,"why is 'she'.....?"

But "why do I care? Why does it bother me? Why do I need to label her? Why am I obsessing over a girl I am not interested in?"

If you have to focus on anything, focus on those questions.
I hope your able to get over her and move on. All the best for your future.