The origination of the mechanism of diffraction nonreciprocity of counter-propagating waves in a gas ring laser is considered. Formulas are obtained for the differences in their frequency and intensity, whose dependences on the mismatch coincide with those derived previously and agree with experiment. Explicit expressions are derived for the differences in frequency and intensity of counter-propagating waves in relation to the dimension of an aperture stop and the cross sections of the Gaussian beams incident on the stop.