There is nothing amazing in that. When you deduct the sum of two numbers, you will get 9 or multiples of 9 i.e. 18, 27, 36,45,54,63,72,81,90,99. The sign for all these numbers are same. Every time you click a new sign comes for all these numbers to make you feel that the what you guessed has come. Please try now. To confuse you same sign is given to some other numbers also.