It's a math trick.
When you add the digits of any two-digit number and subtract the result from the original number, you will get a multiple of 9 as the answer. That is, 9, 18, 27, 36, 45, etc.
The chart with the symbols just has to have the same symbol at each of these numbers. The rest are not relevant. They are just there to distract you.
For example, in the chart shown on the website, there are 95 numbers with various symbols next to them. The same symbol appears next to all multiples of 9.
Each time you play the game, there is a different version of the chart to make you think symbol selection is random.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan