...and to be honest I have no idea why someone mentioned the underexposing problem only with 6d while it is and was present in 5d2 and 5d3 as well. Maybe 6d underexposes even much more.
Funny you say that because in this side by side comparison a completely different conclusion is found:
Strange thing, however they should use an external meter as reference no a camera's one. I sell my 5d2 but I think it underexposed in similar way that 5d3 (-1/3 - 2/3)...I did not have D800 but it should work like other Nikons. However the test you've posted shows ineed a the different conclusion.
Please make a simple test-take few Nikon DSRLs, Canon, and ie. Sekonic Lightmeter-check the metering on ie. white wall-the histograms will be completely different in terms of exposure (histogram peak). I checked it some time ago with 2x5d2, 5d3, d70, d90, d300, d700 and sekonic. When I switched to Canon and just thought that my camera's meter was broken, cause I was used to brighter images in standard exposure. Nikon/Sekonic were ideantical giving a histgoram peak in center. Canon's peak were biased to the left ( ca. -2/3 or -0,5 EV).
A also found that there are some lens which gives a perfect exposure for my bodies - 85mm 1.8. The histogram peak was perfectly in center. All my other lenses gives different results. Why? No idea, aperture maybe ?