Why is the 6D named the 6D?

If you think you have Canon's naming system figured out, well, congratulations.  You say that within a product line higher numbers mean higher specs (or something like that), but certainly the 5D is lower-specced than the 1D cameras.  I don't know what I'd expect from a camera named 4D, any more than I understand why the 7D has that name.  It doesn't really matter, does it?  There aren't so many cameras that it's difficult to keep them straight in one's mind.

FF