5DmarkIII underexposed by 2/3 stop

Here you have white in all parts of the subject: an underexposure with this kind of scenes is pretty normal with any camera! As you may already know, reflected-light meters evaluate the exposure by considering the scene as if it was 18% gray. If you meter something brighter (like you have done), the camera will make it look like 18% green. In modern cameras, like the 5DIII, the camera is partly able to recognize these situations and, thus, it can compensate accordingly, but just to some extent.

Regards,
Alberto

--
http://albertoferrante.name
http://500px.com/albertoferrante
http://www.flickr.com/photos/albertof