Doesn’t explain my experience with Stages gen2 single sided against Kickr, and gen3 left/right. I guess we can hand wave gen3 away by saying Shimano Hollowtech and right side power (in)accuracy 
Force and Red cranks might use slightly different spider designs, maybe, or I’m thinking of the 8 bolts. No matter, I’d guess the L/R estimation algorithm is similar/same.
I dunno, not trying to go off into the weeds. Total power and power curves seem close enough. Rally pedals seem fine so far in my sanity checking. I’ll continue dual recording a couple times a week, just in case, for another week or two.