I think it’s because there are a lot of different things being called “live rock”. CaribSea life rock is called “live rock” at one of my LFS. They have it in their tanks with fish and corals and everything and I’ve seen Aiptasia and pineapple sponges on it so you’d know you were getting those hitchhikers. A lot of organismes (including Aiptasia) start out small enough that you can’t see them with the naked eye so it’s also possible to get rock that looks bare and then a few weeks later the stuff that was already starting to grow there is finally big enough to see. It’s “live” because it is in saltwater fish tanks and colonized with nitrifying bacteria.
My other LFS has their rock in rock only vats. I’m not sure where their live rock comes from and if it’s the CaribSea stuff that’s been cured (in a fish and coral free system I wouldn’t expect many hitchhikers if any). Then you can get aquacultured live rock from places like KP Aquatics or TampaBaySaltwater. This is rock that was either mined from land or “manufactured rock” that has been sitting in the ocean for some number of years. It will be covered in all sorts of visible hitchhikers and also some that don’t reveal themselves for a couple of weeks either because the organisms are growing big enough to be seen or because they were hiding in holes in the rock.
