Why are there differences in parenting because of gender differences in both parent and child?

Old stereo types honey. Sugar and spice and everything nice for girls, snails and puppy dog tails for boys. It's an old addage handed down through the years. The surprising thing, is that it keeps going. Only now, moms are starting to be harder on the girls from their own experiences and men are starting to lighten up on their sons because of how they were treated as children. It's a weird cycle, but decade after decade it continues.Why are there differences in parenting because of gender differences in both parent and child?
I've always wondered myself. So I'm as confused as you are.