i think it's the difference between a healthy average woman and a woman who trains. I briefly dated a former figure girl who had abs. The Mayo clinic will deal with averages.
Equally, as those who train will know, there are a LOT of stories of female Olympic athletes and female bodybuilders who have got down to low body fat levels and have all reported issues with the menstrual cycle.
I think it's a middle ground here on EVO we need to look at. Women here must be interested in health, training and so will have a lower but healthy percentage of body-fat (or at least aim to).