Design output is not correct for Pipe PIP section

Using pipe section (PIP) resulted in incorrect design output. For example PIPS12, while HSSP12.75x0.375 giving different and more accurate result. As for using both, STAAD using AISC H3 equation for HSS section, since PIP is missing the C - torsional constant, it is misleading process, and STAAD should disable PIP design if they don't consider the correct C value.