An Interesting Puzzle

From Sykes Writes:

. . . try this, and see whether you can figure out what is going on.

The answer, courtesy of our genius son, is below in white type. If you swipe the cursor over it, it should change color and you should be able to see the answer.

Note that each time, every number that is a multiple of 9 is the same symbol. And your final number has to be a multiple of 9.