The aim of this work is to give an estimate of the magnitude of the error caused by the neglect of the interference effect and inaccurate consideration of elastic scattering in the case of reflection electron energy loss spectra. Different theoretical approaches to describe electron energy loss processes, as well as the electron trajectories, have been investigated in the case of a Si sample using different primary electron energies (800 and 2000 eV) and geometrical configurations. Through the comparison of the results of the different theoretical models with the available data derived from the experiment, it is shown that the error caused by the neglected interference effect is considerably smaller than the error caused by the inaccurate consideration of the effect of elastic electron scattering, even at relatively low (800 eV) primary electron energy. Furthermore, the results of the theoretical calculations presented here show much better agreement with the data derived from the experiment than the earlier works, in spite of the fact that the only input parameters of the calculations are the dielectric function of the sample and the corresponding differential elastic electron scattering cross section.