Have you observed that the STAAD section properties for pipes differ from the AISC tables? It seems like STAAD calculated the pipe sction properties based on OD and ID and did not account for the reduced wall thickness (0.93 times the nominal wall thickness).
I assume you are using the Pipe (PIP) designation shapes. The area and inertia values for those shapes are consistent with the older codes that commonly referred to them, and hence have the larger, unreduced values you are seeing.
We suggest using the HSS Round shapes instead, as those have properties which are already reduced for mill tolerances.