[dglenn]

By using that formula any number you choose, the resulting formula will be a multiple of 9 (9,18,27,36,45,54,63,72,81) and never have a value greater than 81. If you look at their chart, all of these have the same symbol

99: 99-18 = 81
98: 98-17 = 81
97: 97-16 = 81