BlueBomber jtheat even mentions hearing a second one and that was his first encounter.
If you want to compare them to another animal think gorillas, the alpha male will make any threatening behaviour while the females and young hide.
It's just the way of things, that's why even a silencer wouldn't work. How do you know what's staying silent in the bush within easy reaching distance to you? You may see one it doesn't mean the others aren't there.