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Canon's cameras in general seem to underexpose a bit. I've noticed on my students' rebels, and XXDs and suspect it is a safety feature, so that the inexperienced photographers don't blow out the whites completely. It also used to be the case with the 1 Series cameras prior to the Mark III. My IDII and IIN were always set to +1/3 EC, otherwise it underexposed.
The III and IV exposed differently - they basically nailed it. The 1Dx has a puzzling AE system, in that it will often hit the exposure right on in difficult lighting situation, but under some conditions, it seems to have reverted to the more traditional Canon slight underexposure. One caveat: about the only time I'm using AE is when the sun is popping in and out of clouds and it is changing so rapidly I find myself paying more attention to switching settings than following the action. Mostly I use manual exposure, so my experience is somewhat limited.
Are you using Lightroom to process? Unlike earlier versions, in version 4, the Lightroom defaults upon importing no longer increase the brightness (exposure) by +25 or darken the blacks, so your files are more true to the camera's exposure.
I have a different concern about the rendition of the exposure on the 1Dx: the histogram is not WYSIWYG. I'm setting manual most often by using the histogram, and I find that it isn't as reliable as earlier versions. Since I've also noticed some issues with overexposure (such as metering for the face of players in helmets and blowing out other areas) possibly affecting the AF, I'm trying to get a handle on it. For example, with the earlier 1 series cameras, I knew exactly what level of "blinkies" the camera would tolerate to get the skin perfect. It's very different with the 1Dx. When I meter off the grass now I don't have to stop down 1/3 stop, so it's clear the meter is reading the scene differently.
Where this really is a problem is trying to use the histogram to set exposure in a backlit situation. I'm wasting time in post correcting things that would have been perfect with the Mark III and IV, so would really like to get a handle on what changes are driving the meter.