OMG! How can they know? 

ya dat didnt work i tryed it 3 times soz !!!

Mine was different to the one it showed me :(

did it so many times and they were all right! spooky!

Not to spoil the magic or anything, but it's all purely mathematical. Look at this:
Take the number 99. Add 9+9=18. 99-18=81
Take the number 98. Add 9+8=17. 98-17=81.
Take the number 97. Add 9+7=16. 97-16=81.
So in the end there's only 9 numbers that your final answer can be: 81, 72, 63, 54, 45, 36, 27, 18, and 9. And if you check, all those numbers have the same symbol. Tada - Magic Crystal Ball isn't so magic after all. Sorry.

wow! sweet!