I found it interesting that men were 12% more accurate and only 4% slower when throwing with their non-dominant hand which does suggest that maybe there is some genetic difference between men and women.
I don't think it suggests that. It seems to me that if you're not accustomed to throwing a ball with one hand or the other, then you'd aim more carefully before throwing, and that could lead to greater accuracy. It seems paradoxical, but it's actually pretty commonplace for people who are just learning a skill to take more care to do it right than people who are accustomed to it and whose overconfidence about their skill makes them be less careful (for instance, people who've learned to drive fairly recently are more careful about basic safety practices like checking their mirrors and signaling and the like than veteran drivers often are).
So on the one hand, girls/women in our culture, who'd tend to have less practice throwing a ball in general, would aim more cautiously before throwing, leading to greater accuracy; while men asked to throw with their non-dominant hand would also be taking more care to aim. So in both cases, the difference would be a matter of experience.
(There's also the fact that a sample size of only 20 people from one city isn't really large or random enough to draw firm conclusions from. A difference in accuracy of only 12% could fall within the error bars. Conduct the same test with a different group of 10 men and 10 women and there might be no measurable difference in accuracy, or a difference in the other direction.)