Canon does not claim that these 95% are exactly in the middle, so that you would get the exact amount of slack on all 4 sides of the image.

From the reports so far, it seems that in most cases the reality is closer to the image being flat with the bottom (e.g. no slack there) and having some slack on the left, top and right sides.

Aligning the viewfinder with the sensor seems to be a difficult task when producing those cameras and it seems that Canon goes with the pilosophy that it's okay to have more shown in the actual image than through the viewfinder, but to never cut anything from the image that you originally saw when taking the shot.

Personally I prefer this approach as opposed to one where I lose part of what I see through the viewfinder. I do some cropping on most pictures anyway. Also with knowing how my camera's viewfinder and the sensor are aligned, I usually can easily make up for the discrepancy while composing the picture.

(nubui)