Bone structure.
The reason most women throw differently to men may be connected with the �carrying angle� of their arms. The average woman�s arms are more bent at the elbow than a man�s. This may help to cause the stiff downward motion in ball-throwing that men find so hilarious.
Most women also are knock-kneed. Their legs are built in the same bent angle as their arms. Men�s legs are usually straighter, like their arms. That�s why men are assumed to run straight and �gracefully� and women �awkwardly�, throwing their legs in an arc. It�s why, too, women have a harder time with balance and are more prone to tumbles - a fact for which Mrs O is renowned in these here parts.