Thanks for pointing this out. I think this should have been made clear in the description of the task. I used Python's built-in function variance and got the 'wrong' result, after that I checked manually and confirmed my variance was correct, so I gave up, assuming it was a problem with the Kata. Turns out I was off by just a factor of 11/12th. Very confusing description.
Not a suggestion.
They are asking for VARIANCE. not standard deviation. calculate accordingly. also, note they want 'POPULATION' variance...not for a sample size. So, the denominator is NOT reduced by 1.