I have an olympus EPM1 and shoot in raw. Whenever I use ISO above the lowest of 200, I need to adjust the "exposure" setting in Elem 10's raw importer to the amount the ISO was raised in order to get the correct exposure.
In other words, if I shoot at 800, Adobe raw importer will show the image 2 stops underexposed. The attached image shows a picture shot at 500. Adobe raw converter (above) shows it inderexposed but Olympus Viewer 2 (below) shows it correctly exposed.
As you can see, ACR sought not to clip the highlights in the way that the Olympus software did.
This is perfectly normal, expected behavior.
ACR is not designed to emulate the camera manufacturer's software at all but to give you what ACR considers to be the most natural rendering. It provides you with a starting point to make your optimal adjustments.