Actually, it's not weird that you always get 9 at all, but only if you know a little basic number theory. In any numerical system- binary, hexidecimal, or whatever- continually adding the maximum possible digit will always yield a new number whose digits combined equal the original number. In a base ten system, the highest digit is nine.
And that is obvious if you think about it, because whenever you add nine to something, you are adding one to one side of the number, and taking one away from the other. So if you look at nine like this:
09
You will see if you add 9 you effectively add 1 to the 0, and take 1 away from the 9:
18
And then if you add 9 again you add 1 to the 1 on the left, and take 1 away from the 8 on the right
27
And so on, 36, 45, 54, always adding one to one digit and taking one away from another, so the two digits will still always equal the same number, where you started at.
So, as the question basically asks for a multiple of 9 at the start, that is the same as adding lots of 9s together. So the digits always equal 9.