Look at the black patches at the bottom of the frame to see the difference (between the circuit board and the grey card).
True, but on 99,9% of (my) shots that doesn't matter because the shadow resolution on Canon's current sensors is rather low, so if you see a difference to the 5d3 that's probably the part you cutoff with black anyway.
Why would someone do these things . Wouldn't it make more sense if the more expensive product is also the better one?
Simple - the 7d was released in 2007 (!), the 5d3 in 2012 and the 6d is just a quick defense against the Nikon d600.
And imho a more expensive product does *not* need to be "better", because "better" is relative to individual preferences - Canon is putting too much weight into a "linear" product progression as it is, that's why they did severe and unnecessary cuts to the 6d (1/4000 shuter, 1/180 x-sync, ...). It would be better if "more expensive" would just mean more "pro"-oriented like dual card slots or stellar tracking for pro sports shooting.