Olympus E-PM2 Actual ISO vs IS0 set in camera?

In a digital system, ISO is a nearly useless kludge, a way to adapt a concept of film chemistry where it really doesn't belong. In film chemistry, ISO and ASA actually referred to how much light the photosensitive emulsion had to be exposed to in order to achieve a given density. But in an electronic system, there is no sensitivity other than the sensor's base sensitivity, and there is no intrinsic brightness to the data. There is no density in the way a film negative can have a measurable density. The signal isn't anything until a computer, translating the recorded signal into something human-readable, assigns a pixel saturation level an RGB number. This is done in one of five methods as specified in ISO 12232:2006, and as employed in cameras, only applies to resultant JPEGs in the sRGB color space (so for those of us who shoot RAW and edit in ProPhotoRGB, the concept is already failing us).

The only real reason we still have ISO in cameras is because we got used to it over a century and a half of chemistry-based photography, and we still haven't gotten over ourselves despite the fact that everything has changed. Electronic signal capture has almost nothing in common with recording of a latent image on a chemical medium.

ISO's only real point of being handy is the way CMOS sensors generally have reduced read noise at higher ISO settings, and thus slightly better signal-to-noise ratio at those higher ISO settings.

As others have mentioned, the method used by DxO has nothing at all to do with the standards laid out by the International Organization for Standardization.