First of all, thanks for doing such a thorough test!
It seems as though EV1, EV2, EV3 & EV4 is where the problem is. Below EV1 the focus assist light comes on (and focus is good) and above EV4 the focus is not off by much. It might be good to concentrate on EV2 (red in the chart below) and EV3 (blue in the chart below).
Pondering the problem, my first theory is the focus sensors stop working on some color wavelengths between EV1 & EV6 and eventually all color wavelengths.
As the light dims and wavelengths drop off the focus sensor, it plays havoc with focus accuracy. Under certain low white balance situations (maybe below 5000k or so) the problem is worse because those are the wavelengths dropping off first. If there is some bright white in the frame it can also make matters worse because the focus assist light won't come on as early.
My second theory is, not all focus sensors drop off color wavelengths at exactly the same EV value. The hardware has tolerances and some are better than others. Therefore some K-5's may perform differently than others at these crucial cut-off points. Furthermore, then getting the firmware to adjust for the fall-off and focus change is difficult, because it's never exactly the same.
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I would like to see multiple tests with different K-5's done in the EV2 & EV3 area (as well as EV 8 or so for accuracy comparison). Also, the focus distance is important. I think between 3 and 5 feet (1-1.5 meters) is where the problem is more pronounced.
It would be interesting to see if some K-5's actually handle these low light settings accurately!