By this I assume you also say that your knowledge of logic doesn’t allow you to come to conclusion that if Suttas describe unequivocally that senses stop to work definitely in immaterial states, before arupa attainment there is rupa which obviously has to be perceived by senses.
Why equanimity which is diversified (equanimity towards sensory experience) should be surmounted by equanimity that is unified this perhaps you can understand following simple logic: it is much more sublime experience.
But when Sutta describes equanimity which is unifed as immaterial states not jhanas, your logical skills aren’t good enough to make inference: it must be so because below immaterial attainments still there’s diversity of sensory experience.
Interesting 