"The camera’s sensor does not give equal weight to all tones. In fact, your digital sensor is heavily weighted to the brightest areas in your photo. (...) Taken another way, the camera has a ﬁxed number of numeric values for describing the brightness of a pixel. Fifty percent of those numeric values are devoted to the brightest f-stop in your photo. Each successively darker f-stop receives one-half the number of the f-stop ahead of it, until the shadows receive only a small sliver of the total possible values. This is important information, because all detail in your photos is a result of subtle differences in tone and color between adjacent pixels. In the shadows, where fewer values are available to describe these differences, it becomes more difficult to retain details. Underexposing photos drives more of the information contained within a photo deeper into the shadows, causing a loss of detail and an increase in noise (unwanted color impurities) in the photo."
(Taken from Perfect Digital Photography, 2nd edition, hopefully not infringing any copyrights.)
That is correct. If you want to know the reason is quite simple. Humans see light in an approximately logarithmic fashion (as if we were taking the log (base ~2) of the actual light we see). (We also hear in a logarithmic fashion as well.) This is very useful to us since it means we can see when there are just a few photons, and when there are tens of thousands of times more photons per until area of our eyes - and yet it looks to us like it's only a few times brighter.
Sensors are linear, they just measure the approximate number of photons per pixel. So if your images has 9 stops of dynamic range, the brightest stop has half the the available data. And the darkest stop only has ~0.2-0.4% of the data.
Exposing to the right (as long as you aren't blowing out the highlights), is a very good idea if you want to have more freedom to play with your images afterwards - since you'll have much more data in the shadows. Just one stop of "overexposing" will give you twice the shadow detail.
However I digress, and still want to know what's going on with those 6D long exposures...