Basically the numbers that you see are only decimal approximations of the real binary values stored in the computer's memory. With calculations involving decimal fractions these values can differ by a small amount even though they look they same when you look at them in the command window or variable viewer. However they will fail an equality test, and so are not considered to be equal, thus ismember does not match them.
Information on Floating Point Numbers
And some external links on this topic:
