Sorry to dig up this old discussion, but the topic is active again in other threads, so maybe this one is worth bringing to peope's attention.
As I have mentioned in numerous times, Japanese camera makers are more or less required by their industry association CIPA to use ISO Standard Output Sensitivity as the primary method of calibration of the ISO sensitivity settings on their cameras.
To which Bill Janes replied
The D800e uses REI (recommended exposure index) for its speed rating and I understand that this is also in compliance with the CIPA standards. The REI is arbitrary and the maker could assign whatever value they wish, so that standard is not lilmiting. In my own tests with the D800e exposing the target at ISO 100 according to the light meter reading gives a sensor saturation of 12.2% which is in accordance with the Ssat standard.
My original statement needs some refinement. As far as I can tell, the flexibility of "REI" is there simply to allow flexibility in multi-zone light metering algorithms; insisting on SSOS would restrict cameras to some standard averaged brightness metering. In practice it seems that basic metering modes do more or less follow SSOS. In fact, if a camera were to deviate significantly form this, its handling of simple low contrast scenes would produce out-of-camera JPEGs significantly lighter or darker than from other cameras, and customers would notice, and complain. In other words, for basic metering, or metering on a subject of fairly uniform brightness (as with spot metering), market forces keep in-camera metering close to SSOS, with placement of metered mid-tones at a JPEG level near 112, which is just under 18%.
Also, allowing for the highlight roll-off typically used in default JPEG conversion tone curves, it makes sense that mid-tones are placed a bit below 18% of FWC (or maximum raw level): a 1/2 stop of roll-off requires placing them at about 12.5%, close to what you observed with the D800e. This is also what the ISO 12232(2006) standard hints at when it defines SSat: it mentions adding 1/2 stop of headroom.