That is because they don't practice at 100 yard distances, isn't it?
I'm always surprised at people shooting or sighting in at 25 yards, that's a good distance to retreat from.
On horseback, two forces closing at a gallop, the time elapsed between the hundred yard shot and hand to hand contact would be 5-6 seconds. With a moving platform and a moving target, closing speeds of ~40mph, or ~58' per second, I imagine all hits to be more or less accidental. With each ~20 stride of the horse though the odds of getting a hit would increase.