The numerical densities of Si and O atoms for 3.5-6.5-nm-thick silicon oxide films grown on Si(100) at oxidizing humidities of 0.5 ppb-500 ppm were determined by Rutherford backscattering spectrometry. Especially, the numerical density near the oxide-Si interface was rigorously investigated by comparing the results with the previous volumetric density evaluations using charged-particle activation analysis (CPAA). It was confirmed that excess Si atoms relative to the stoichiometric SiO2 composition exist near the interface and their number decreases with decreasing humidity. In addition, this humidity dependence was similar to that for the "slight discrepancy of the oxide thickness" (DeltaT(ox)) found by CPAA, which was strongly correlated with the humidity dependence of the volumetric density, device reliability, etc. Therefore, it is possible that all the humidity dependence has a common origin: Dehydration results in a reduction of the excess Si atoms near the interface, and thus produces a decrease in Si dangling bonds.