<p>So whatever the hell it is you pick will be a multiple of 9 (that is less than 9*10=90 and greater than 0, because 0<a<10). Now look at the multiples of nine in the table: they're all the same letter.</p>
<p>Ok, it's late and I'm on not much sleep, but why does that matter? Yes, it severely limits the number of possible letters but I've gotten different letters different times and it's still gotten it right.</p>
<p>Well if you're always going to get a multiple of nine, and each new table specifies the same letter (say, z) for all the multiples of nine, then they know that the letter you looked up is z...</p>
<p>Notice that the symbols keep rotating with every click of the mouse. The symbol that appears is always a multiple of 9. ( Watch the symbol for 9 each time you click it)</p>