The bending potential function of SiH3NCO, including relaxation effects, was calculated at the DZP/QCISD and TZV2P/QCISD levels of theory to establish reasons for the disagreement between the ab initio and experimental equilibrium structures. The microwave, infrared, and Raman spectra were reinvestigated using this bending potential. It was shown that the asymmetric top geometry with a double-minimum bending potential is not a unique interpretation. Two alternative assignments for the microwave spectra were put forward, based on a symmetric top structure and a strongly anharmonic bending potential. It was shown that including triple excitations in the ab initio calculations at the TZV2P/QCISD(T) level generates a small barrier in the potential. Also, the importance of allowing for relaxation in the analysis of the microwave spectra was demonstrated, as this substantially decreases the barrier size. These results bring the experimental and ab initio structures closer to each other. The effects of small-amplitude motions and the existence of a small internal rotation barrier are also discussed.