We investigate the influence of inelastic processes on reflection high-energy electron diffraction (RHEED) oscillations by recording energy filtered RHEED intensity oscillations during homoepitaxy of (001)-oriented GaAs. The results clearly show that the dominant inelastic scattering process, plasmon inelastic scattering, does not influence the phase of the oscillations. It cannot therefore account for an independent process contributing a phase to the oscillations that is different from elastic scattering. As an alternative approach, we investigate a basic coherent scattering model introduced by Horio and Ichimiya. We compare its predictions with experiments in the one-beam condition for both GaAs and AlAs(001) homoepitaxy. The average crystal potential required for the fits can be determined independently by Kikuchi line fits, yielding a value of 10.5 +/- 0.5 V for both GaAs and AlAs. This allows us to reduce the number of free parameters in the model to only the layer thickness. The theoretical fits of the model to the experimental data yield different layer thicknesses that are in good agreement with the surface reconstruction thicknesses for GaAs and AlAs. We therefore conclude that the phase of RHEED oscillations is determined by the surface reconstruction forming on top of the growing layer during crystal growth. This new model explains many experimentally observed RHEED oscillation phenomena in a unified approach.