There are a small number of people out there with wrong chromosomal numbers, and there's also things like Androgen Insensitivity Syndrome.
There's also just people who've got unusual body types.
Gender is far from binary of course, but even sex itself can't quite be shoehorned neatly. Incidentally, even if the IAAF rules that she can't compete as a woman, that doesn't change the fact that she self-identifies as such and should therefore still be considered a woman.
The whole thing is quite sad and degrading for all concerned, and this question and the urge to pigeon-hole has a long history of causing pain and suffering to people. There's a lot of stories of surgical remodelling, psychological trauma, etc, of intersex people.