It is assumed that in a number of cases the typical ’depressed semicircles’ and other distortion phenomena are caused by a distribution of parameter values due to surface inhomogeneities. A model is proposed to treat these distortions in terms of the distributions of the parameters (average, variance, covariance and higher moments). It is shown that the phase angle of the impedance is invariant with the distortions caused by the surface inhomogeneities. The mathematical results and their consequences are discussed and demonstrated on simulation diagrams of two frequently used equivalent circuits.