Every multiple of 9 (9, 18, 27, 36, etc.) has the property that its digits will always add up to a multiple of 9.
For example, the number 4,968 is a multiple of 9 (552 × 9), and its digits add up to 27 (3 × 9).
Because of how the problem is stated, with four-digit numbers, the sum of the digits will always be 9 or 18 or 27.
Adding 1 + 8 or adding 2 + 7 (as specified in the problem) again gives 9.