The things you gotta do to keep your video memory down when you're making a game on 256mb of VRAM. Gamedev is full of cool tricks like this.

Those cheeky bastards, I would have NEVER known

And the clouds are the same rock, just textured white.

Probably taking advantage of geometry instancing

They did a good job.

I was building maps in Halo Reach and noticed the same thing. There were like one or two rocks to choose from. I wondered if anybody would know I was using the same rock repeatedly.

This is standard practice in virtually all video games ever..