That's usually because the security code consists only of #s between 1-6. Unless I'm mistaken, you will never find a code with a number 7,8,9,etc. in it.

I'm not an expect on 2001 OEM CD players, though... for all I know, you are staring at a slip of paper in your hand reading "79897" or something. But as far as the older decks go, it's a combination of numbers from 1-6.