It might be, you're absolutely right, but it isn't. It just doesn't work that way, for no particularly good reason, other than that's the way I envision it to be
The Nameless Rock could just as easily be a water site, it's a tiny island in the middle of the ocean, the peak of a volcanic mountain, rich in gold and rubies, poking it's way just above the waterline. It could therefore be an earth site, or even a fire site.
It just happens that it's an astral site. That's how things fall out.