Ok this is what the experts say -

It's perfectly normal for both girls and guys to have hair in their "butt-crack" - the area around the an*s and between the cheeks of the buttocks. In fact, most people have hair in this area. When girls go through puberty pubic hair grows around their genitals — over the pubic bone and around the vulva and an*s.

There you have it - its there from over growth.