We present a model which describes the appearance of excess and deficiency features in electron backscatter diffraction (EBSD) patterns and we show how to include this effect in many-beam dynamical simulations of EBSD. The excess and deficiency features appear naturally if we take into account the anisotropy of the internal source of inelastically scattered electrons which are subsequently scattered elastically to produce the EBSD pattern. The results of simulations applying this model show very good agreement with experimental patterns. The amount of the excess-deficiency asymmetry of the Kikuchi bands depends on their relative orientation with respect to the incident beam direction. In addition, higher order Laue zone rings are also influenced by the same effect.