Why are there patterns to male pattern baldness?

The hair on the very top of my head has been falling out for a few years now. So I have a bald spot, oh well. But why is it THOSE hairs? The hairs on the side of my head seem to be growing as enthusiastically as they ever did.

On the other hand, some guys go bald by having their hairline recede into their head until finally, I assume, it meets up with their neck and they’re all forehead. That’s not happening to me. I guess I’m Type 1 Balding, as opposed to Type 2.

But why are there those two specific ways of guys going bald? Why don’t you ever see a guy having the baldness start over his left ear and creep around his head side-to-side, or something like that?