ISO 100 is the key. I do not have any other ISO 100 shot. I verified shots wityh perhaps all other ISOs, and all get +0.5.
I suggest you to make two shots of the same scenery (perhaps the Stouffer), with the same exposure, but one with ISO 100, the other with ISO 200, and compare their histograms. There should be some clipping as well.
I went through my test shots and found what you want. The exposure is the same for both shots, 1/15 s, f/8. As the histograms show, the same values are written to the raw file for both exposures, but they are processed differently by ACR according to the BaselineExposure value, which is -0.5 for the ISO 100 and +0.5 for the ISO 200. If I cancel out these values by adding +0.5 EV in ACR for the ISO 100 and -0.5 EV for the ISO 200, the results are more or less the same.
BTW, Rawanalyze reports ISO 200 for the ISO 100 exposure.
Histograms, 100 left, 200 right:
ACR defaults, 100 left, 200 right:
ACR Plus 0.5 EV left, Minus 0.5 EV right: