I feel like I should know the answer, but for the life of me I cannot make sense of it. 

If I create a very simple NSC model consisting of a 1 Watt source reflecting at normal incidence off a flat mirror and then absorbed a by a detector also at normal incidence, the power absorbed by the detector will vary whether I have “Use Polarization” checked or not.  With use polarization, the power is ~0.946 Watts, without it is 1.0 Watts.

What gives?

I am running an older version of Optics Studio but can upgrade (22.1.2).