From that site: caryatid - a supporting column sculptured in the shape of a woman.

The male counterpart is called an atlas and telamon (plural Atlantes, Telamones). Atlas was the giant who supported the sky, and atlantes are often depicted so as to suggest the strain of carrying a huge weight on the shoulders. The female equivalent--the caryatid--is, in contrast, usually shown standing serenely upright.
http://www.xrefer.com/entry.jsp?xrefid=142799

There is a more pragmatic reason for the difference in posture. The narrowest part of the standing male, the neck, would simply be too weak to support any significant weight. But in a sculpted standing women, with hair flowing down the back of the neck, the hair and neck are in fact a single piece or stone, wider and stronger and thus able to support much more weight.