Well, the average outdoor scene supposedly integrates to 12.5% gray and that is why light meters are calibrated to that number.

18% gray was, IIRC, picked because it is "middle gray" and the progression of grays is a geometric one, with each step being twice or half what the previous one was.

If you carefully read Kodak's instructions on how to use an 18% gray card, you will see that it accounts for the difference between the 12.5% and the 18% numbers.