Bragg scattering of atoms from a standing light wave exhibits Pendellösung oscillations in the scattering probability. For second-order scattering, we have recently observed significant deviation of the oscillation frequency from the prediction for a pure four-photon-type process. To give a quantitative explanation for this behaviour, we present an analytical calculation which takes into account lowest energy momentum states. Our calculation suggests that a transition between a four-photon-type process at low light intensity to a two-photon-type process at higher light intensity is responsible for this behaviour.