All multiples of 9 have digits that add up to 9. This is something kids learn. Why does it work? 9 is 10 - 1, so every time you add another nine the tens digit goes up by one and the ones digit down by one, maintaining the sum of digits. Similar thing happens at three digits, four digits, etc, because 9 < 10, no digit other than the ones digit ever goes up by more than 1, and a 9 becomes a 0 when you carry a one.

That this pattern didn’t swamp the results makes me wonder what kind of bug the author has in the scanning code.

[edit: seems I forgot about situations like 9 x 11 x N, where the digits can add up to 18, 27, etc due to all the 8 and 9 digits, so the algorithm will only find multiples of 9 with lots of small digits in it.]