trying that few time i got how they do this.
but anyway thats nothin much to be unbelievable in here cos its simple thing.
what happens here is there's only few numbers that come as the final number (as answer to that formula) when u think of any 2 figure number.those final numbers are 81, 72, 63, 45, 27, 54 , 36 ......
ex; say if u take 99 then final number = 81
even if u take 98 u get the final number =81
even if u take 97 u get the final number = 81
any number >= 90 will get u the answer 81.
80 =< any number < 90 will give you the answer 72.
so all those numbers have the same icon at one time, so what ever number u thought they(probable answers) all has the same icon, so that is shown inside the ball.
trick is they change the icon of each number at each try