Best answer so far is Schnauzer's!

a. who wrote the Wiki entry?
b. Could be that it has to do with the meaning, history and characteristics of the trio.
e.g.: anynumber multiplied with 9 can be collapsed into itself numerically.
9 times 107 = 963 to 9 + 6 = 15 = 6, 6 + 3 = 9
9 times 88 = 792 to 7 + 9 = 16 = 7, 7 + 2 = 9
9 times 7215 = 64935 to 6+4 = 10 = 1, 9+3 = 12 = 3,
voila ... 1 + 3 + 5 = 9
Attention: numbers always have to be added in sequential pairs!
Folks who are into numerology and other stuff choose numbers, dates according to things they want to express - it's an age old "science" and could be knowledge from beyond our reach which flowed into our tangible system.