 As can be seen, the lobe tilts downwards towards P' which is not the same as the on-axis listening position P.

This is one of the reasons for physically offsetting the mid-range and tweeter such that the tweeter is physically behind the mid-range, or tilting the speaker upwards, to achieve an on-axis response that is coincident with the physical on-axis plane.This process is known as time-alignment.Other ways to time-align are to introduce a phase shift in the tweeter signal (lag or lead, depending upon the offset between the drivers) so that it mimics the physical offset.