That's interesting!
My only guess would have been that it had something to do with what side you mounted a horse from.
I remember reading an interesting paragraph once, pointing out that the width of railroad tracks was set to be the width of a wagon drawn by two horses: and the size of the space shuttle was determined in order to make it possible to ship it by train. The point being, that, even in the space age, we are still basing our engineering on the average width of a horse's @ss......