That messed with my mind for ages but I worked it out.
Any number you pick and put through the calculation with always give you a multiple of nine.
Pick 10 to 19 and you'll get nine
Pick 20 to 29 and you'll get eighteen and so on.
Each time the website changes the symbols around but keeps all the symbols on a multiple of nine exactly the same up to 81, because 90 and 99 are impossible to get.
Check it out, 9/18/27/36 etc will all be the same so the website knows what symbol you'll end up with and is not reading your mind.
Apologies for ruining everyones fun