Simple! If you add together the digits in a two digit number, and subtract the sum from the original number, then the answer must be a multiple of 9. (10X + Y - (X + Y) = 9Y) So all the website compiler has to do is provide the same symbol for all multiples of 9.
A clever elaboration is that each time you enter the chart, the symbols for the multiples of 9 change; but you will note that they are all the same up to an including 81.