See if you can make anything out of this:
I am predominantly a lefty. It is the hand I write with and the first hand I go to for most motor tasks. Baseball or golf swing also left handed. Jumpshot left handed. Here is where it stops making sense... I am much better at dribbling a basketball with my right, and for reasons unknown or forgotten, I learned to throw a disc right handed as well. I also practice Kendo right handed but they wouldn't teach it lefty to me in Japan. At least that part is easy to understand.
Anyway I thought along similar lines because it seemed like I may have greater potential for precise msucle control with my left arm. I haven't put a lot of time into my left handed throw so far, but it is proving difficult to learn compared to throwing forehand with my right arm. In other words, yes, it does take a ton more practice to learn to throw from the opposite side of your body.