Haha that's really nifty, though. I tried to trick it by choosing the same number again but the wee symbols change when you refresh and it got it again...
AH HA! I've worked out how they do it! Ooo, very tricky... I'll write it really small down further so it doesn't ruin the fun but I have to share because I'm so proud, tehehe.
Don't read the end if you want it to stay spooky!
For each lot of ten (90-99, 80-89 etc) it leads to one number. For example 90 - 9+0=9; 90-9=81 or 99 - 9+9=18; 99-18=...*gasp*81!!! Same goes for the 80s and 70s etc. 72 - 7+2=9; 72-9=63 or 75 - 7+5=12 75-12=63!! So, really, there are only 10 answers you can ever get and the same 10 symbols are given to those numbers. But then, to mix it up and think you're getting different numbers, they mix up the symbols each time. *dances* oo yeah, oo yeah, go me, i worked it out.