The difference in strength is not a calculation error, but arises from the physics of sound propagation around the head when not facing the source directly. At high frequencies (short wavelengths), the head acts as a barrier for the ear on the far side. The direct sound from the source does not reach that ear – only the diffracted part, which is much weaker.

This ‘shadowing effect’ by the head is more noticeable in rooms with high absorption and weak lateral reflections, which are not able to compensate for the lack of direct sound at one of the ears. An HRTF (Head Related Transfer Function) example of this case is shown in the figure below.