When you add up the numbers, they are always 45, 54, 72, etc. All the numbers that are a possibility have the same symbol. When you click on "try again" all the symbols are refreshed to different symbols, fooling you into thinking the symbols are random.
Very sneaky. They know that the number will always be divisble by 9, so they put the same symbol in all the 9s, like sockiesmom said. It is pretty cool, and will catch you off guard at first.