The steel design output for a tube section checked per the AISC ASD code indicates an SY and SZ substantially different from the values which are reported in the AISC publication. Why?

In steel design per the AISC ASD code, the elements of the cross section (flange, web etc.) have to be put through some tests per Chapter B of the code. These tests are required to classify the cross section into one of 3 types - Compact, Non-compact, Slender.

If a section is classified as slender, the allowable stresses on the section have to be determined per the rules of Appendix B of the code. For slender "stiffened elements", which is the type a tube falls into, the effective section properties have to be calculated and those effective properties must then be used in computing the actual stresses.

The extent of the cross section deemed effective depends on the bending moment on that section. It is very likely that for the critical load case, the effective properties are less than the gross section properties, which is why you see the reduced Sz and Sy in the output.