Optical spectra of US nanocrystals are interpreted by using both the atomistic tiaht-bindin- method and multiband effective mass theory. Both methods correctly describe the energy splitting between the two lowest optically active transitions and their relative strengths, providing the same labeling of the two main absorption peaks of the spectrum. Our calculations unambiguously show that these peaks correspond to the 1S(3/2) --> 1(S) and 1P(3/2) --> 1p transitions. Both zinc blende and wurtzite-type structures for US nanocrystals are considered. Similar optical spectra are predicted for the two lattice structures. We also study how the spectrum, and in particular, the 1S(3/2) --> 1S(1/2) splitting, is changed by modifying parameters, within the experimental uncertainties, including size and shape fluctuations, surface passivation and spin-orbit coupling. Our results are robust to small variations in all of these parameters.