The bigger picture on equalization

I felt very confused after finish this article. They did not give us too much information therefore it makes me felt frustrate. First of all they talk about measure loudspeaker frequency not only “on –axis” response but also concern off-axis response. They suggest average every audience sit position and tune the speaker as flat as possible. However when we only turn one speaker on to measure the speaker frequency instead of turn both left and right on.  It maybe sound good when we just tune only one speaker on, what about turn both speakers on at same time? Do I have to tune the EQ again? Or what should I do? The second question is the phase issue by looking at the 3 dimensional radiation balloon.  I know a properly gathered balloon will reveal the anechoic response of the loudspeaker at all listener angles but I don’t understand when the article compare between figure one and figure two pictures. I understand the F1 picture but why the Figure 2 will come into phase at two-off axis position? What is meaning “at two-off axis position”?   

I felt the more I learn the more I don’t understand.