I tried it 3 times, and it never got it right. It looks to me that it has a 1 in 9 chance of being right.
it was wrong at least twice for me, also. hard to know how it is supposed to work. i am waiting for robt to tell us that.
It has been right for us every single time. Are you sure your subtraction is right? I'm not being a wise guy. I did subtract incorrectly one time and yelled A-HA! and then double checked afterwards.