I am not sure about the 18% gray thingy. But then practically speaking, I find that an "18%" gray card works. I went one step further and tried with another darker grey object, and it gave me an almost similar reading on my meter too. So at the end of the day, whether its 18% or 12%, I think that practically speaking, when it comes to taking the photograph, it won't really differ or matter much.
How bright a gray card is depends on the angle from which it is illuminated. Let's say an 18% gray card is illuminated from a 45 degree angle - it will give you the same brightness as a 12.7% gray card illuminated straight on.