I have a beam member for which I have assigned a single angle from the American steel table. When I look at the member properties output for that member, the values that STAAD reports for moments of inertia Iz and Iy do not match the values I see in the AISC steel publication for that angle section.
The numbers reported in the STAAD output for Iz and Iy are the moments of inertia about the principal axes of the single angle. The values in the AISC publication that you are comparing them with are most probably the values about the geometric axes. That is very likely the cause of the mis-match.