No kidding. I guess it started when Nikon had the Nikon F in the fifties. Some genius at Canon decided to call their supercamera the "F1".
But then Nikon replaced it with the Nikon F2! Argh, what to do? If they called their next model the F2 also, they would look like a me-too. So probably the same genius called it the F1 mark II. And both companies, sticking to the guns they know, have been following the same pattern since, with the big models. They did the same with the big electronic cameras on the nineties which still used film. Those very rounded ones. Weren't they called D1 or 1D or something like that also? Maybe just "1" and "3".
Of course come digital, Nikon calls their cameras D.... , while Canon calls theirs ....D. It's insanity. And spare me the "EOS" and "Coolpix" and "Powershot" just before the name... Who the heck knows or cares which Canon is a Powershot or not, or why?
Camera companies and many others too would do well to hire somebody with a drop of sense and imagination to name their products. 40 years ago I read of a Canon experimental camera called the Canon Frog. See, *that* name I can remember, decades later.
Despite them being near triplets, the 7D is not full-frame, while the other two are. And quick, tell me: the successor to the 5D mark II, is that the 6D or the 5D mark III? (Both exist, but oddly the 5D mark III is much more advanced and expensive than mark II, while the 6D seems very much like a successor...)