My sister is 'double jointed' and can bend her elbows back at such an angle that it looks almost like a deformity. When she was younger she could easily bend over backwards and touch the floor behind her with her hands.
But to get back on topic, I have observed the "carrying angle" phenomenon of many women and I would say that over 90% of genetic women display this feature and also the ability to bend or curl the fingers back at a greater angle than most men. The latter is a very definitive female characteristic, put to good use when gesturing.
Those like me who were not born with these features can exercise and practice in order to achieve greater suppleness in the joints of the fingers, wrists and elbows, which can help us to conform better to genetic female characteristics. Danae Doyle deals with this issue in one of her helpful tutorial videos.
Louise