This value should be zero, why is it not zero?

    >> X

    X =

        90    60    45   100    15    23    52    30    71    88

    >> xbar

    xbar =

       57.4000

    >> diffx

    diffx =

       32.6000    2.6000  -12.4000   42.6000  -42.4000  -34.4000   -5.4000  -27.4000   13.6000   30.6000

    >> clear sum;
    >> sum(diffx)

    ans =

       1.4211e-14

    >> 
       1.4211e-14

    >>

Answer 1

[Matt J]

You generated the diffx data (and summed it) with inexact floating point arithmetic, as all computers do.