This is a good question and I'm glad you asked it. While not a lady myself I have done extensive research on this topic, and the short answer would be its an evolutionary trait that allows for the quick emasculation of the opposite sex.
Try it for yourself.
Put your hands on your hips with it facing back, the mans way, quickly take you hands of your hip and put up your index finger as if you're wagging your finger at a naughty puppy who has defecated on the kitchen floor.
Now do it again but this time with the thumb facing forward.
You feel the difference right. The mans way is mechanical and awkward. But do it the female way and you have rotation of the wrist and a little swivel....
'Ohhhhhh noooo you dih'int!'