because the bits she may have aren't necessarily indicative of her chromosomes.
IE - Kleinfelters syndrome is where a boy has two or more X chromosomes. He still has a cock and balls but they stay at the prepubescent size for the entirety of that person's life.

Or - Turners Syndrome - where a girl is missing an X chromosome. They are infertile and unless hormone treatment is given - they don't develop at all during puberty.

Those are two of the more well known sex-linked chromosomal disorders (as in X or Y - girls have XX, boys have XY). There are other disorders that it could be - and not many of them are necessarily able to be determined by "looking at what bits she's got".
(Poppet some assembly required., Thu 20 Aug 2009, 5:11, Reply)