There are others who have wondered about the same thing. No definitive answer yet:
http://ask.metafilter.com/mefi/42605
There are also people who say that the word "number" is wrong and should be "letter", but I'm not sure. If we're to think the letters are numbers, then I'd go with Ponrauils explanation, but I also like "there are no numbers there, so it's 0. the next number is 1.".
Ice