We carry out ion scattering simulations to investigate the nature of the transition region at the Si(100)-SiO2 interface. Ion scattering experiments performed in the channeling geometry provide us with a genuine interfacial property, the excess Si yield, resulting from distortions in the Si substrate and from Si atoms in intermediate oxidation states. To interpret the ion scattering data, we first generate a series of model structures for the interface by applying sequentially classical molecular dynamics and density-functional relaxation methods. These models reproduce atomic-scale features consistent with a variety of available experimental data. Then, we design a classical scheme to perform ion scattering simulations on these model interfaces. In our study, we separate the excess Si yield obtained from experiments in two distinct contributions. First, Si atoms in intermediate oxidation states account for similar to 25% of the excess Si yield, a contribution that is fully determined by the population of suboxide determined from photoemission data. The remaining similar to 75% of the excess Si yield characterizes the amount of lateral distortion of the substrate Si layers in the vicinity of the Si(100)-SiO2 interface. The comparison between calculated and experimental excess Si yields indicates that the distortions propagating from the interface into the Si substrate are consistent with interfacial transition structures extending over more than two Si layers, eventually including a disordered bonding pattern. Nearly abrupt interfaces induce distortions in the upper layers of the Si substrate which are insufficient for reproducing the experimental excess Si yields.