Why the rear rotor?
 

As I understand it, the rear rotor will generally have lower compression non a higher mileage engine. I do believe that in the piston world this can be caused by cylinders running rich or lean. I will mention a carbureted I6 here. A rotary generally should be better balance of air and fuel semetrically into both rotor housings. The only thing I can come up with is the exhaust manifold may not flow as well with the rear rotor. If that is the case, would headers alleviate this problem?

Rear rotor runs hotter as it is cooled by coolant that has run through the front rotor housing already. The coolant cools the rotor housings and the side housings as well.

By running hotter side housings the side seals would be longer through thermal expansion at operating temperature. When the engine is cold this means the side seals would have worn to have a larger gap in the rear rotor than the front rotor.

The side seal gaps are most of what we are measuring in a rotary compression test.

My experience is-
Rotor face with good apex seal and good side seal - 80-90psi cranking compression.
Rotor face with good apex seal and no side seal- 30-35psi cranking compression.
Rotor face with no apex seal and good or bad side seal- 0-5psi cranking compression