How do we know he's only fired 10 or 11 times? Answer: We don't. We just assume it, probably because it makes him seem scarier.
In reality, the shooter could have missed 10 or 20 times, too, and everybody who heard it chalked up the sound to a backfire or something else.
All we really know for sure is that he fired these particular shots that happened to hit people. He could be a pretty average shot, and I bet we'll never, ever know about his misses.
Food for thought.