Here's an explanation to your question. This is from the Titus Bicycle website.
This information is for cyclists 5'5 and under.

"When it comes to fit, a true woman specific design will feature adjusted angles and geometry to fit a female’s body dimensions. Aside from a shorter top tube needed for a comfortable reach and increased standover, the seat angle should be steeper to accommodate a woman’s shorter femur length. The steeper seat angle position allows a woman to be positioned properly over the pedals. The average seat tube angle for both road and mountain non-female specific designs is typically between 72 and 73.5 degrees. The seat tube angle on female specific designs should be between 74 and 75 degrees. This steeper seat angle also contributes to better handling. A steeper seat angle results in a longer, more stable wheelbase. Anything that can be done to get the wheelbase a little more stretched out on a small frame will result in a more stable, better handling bike."

http://www.titusti.com/womensfit.html

I don't know if the short femur theory is true for every small woman but I'm
5'1" and all my bikes have the steep sta including my custom frame.
Without the sta there is no way I can get correct saddle setback or (KOPS). Even with a sta I require a zero degree seatpost and my saddle has to be set forward in order to achieve correct knee positioning.