I really do not understand how this is possible - can somebody shed some light on this for me please? Surely if I take each of the following 7 datapoints, subtract it from the mean (x-xbar), and then sum together all of those errors I should be left with 0... But instead I get +- 6.66134*10^-16 (6.66134E-16 in excel).

Numbers:

0.678571429

0.75

0.767857143

0.839285714

0.678571429

0.767857143

0.517857143

(mean = 0.714285714)

Thanks