I'm surprised by the discrepancy in scores. I can also do more than 9-back and personally know people who are considerable smarter than I am (e.g., Nick Bostrom and Robin Hanson). I suspect that the n-back game does not very strongly correlate with IQ, or else that scores in this game can be dramatically boosted by the use of certain subtle mnemonic and visualization strategies, which even intelligent people may fail to adopt.