Reflection high energy electron oscillations were observed for the first time in the early nineteen eighties. Currently many researchers who deal with the preparation of two-dimensional systems use them to control precisely the thickness of deposited structures. However, one may easily find in the literature essentially different approaches explaining the origin of the intensity oscillations. In fact, it means that up to now the question of why the intensity oscillations appear is still open. In this paper a combined approach is used to explain the effects observed for the case when small atomic islands are formed at the surface during crystal growth. In this approach changes of the refraction conditions at the surface and changes in diffuse scattering are considered simultaneously.