haven't worked out the math, but evidently when you subtract the digits of a two digit number from itself, you come up with a limited set of possible answers.
For instance 25-2-5 = 19 and 20 -2-0 = 18 and 30-3-0 = 27 and 31-3-1 = 27
In the page with all the numbers, all the possible numbers have the same item listed.
The page with all the numbers changes each time so you don't keep getting the same item when you start over with a different number. Look at that page and then select a number, do the math and the result will always give the same item.
Ok...the math.
Start with the number 30. The result of 30-3 =27 Then if you add 1 to the 30, you also subtract and extra 1 ending up with the same 27 for an answer. So the Key Numbers should be 9,18,27,36,45,54,63,72,81,90 all the powers of 9.