I see these terms as forked off because each of their proponents needed to carve out their own niche.
I don't see them as very different from each other in the big picture, so I pick and choose parts of what works for me from each.
A clue to this mechanism is the subtitle to Nora Gedgaudas's book Primal Body Primal Mind, which is "Beyond The Paleo Diet For Total Health And A Longer Life." The intended claim is that this book is better than one of the "Paleo Diet" ones, so it would lead to total health (where the "Paleo Diet" has gaps? and longer life, where the "Paleo Diet" wouldn't?)
I doubt Nora herself picked that tagline, more than likely a marketing person did, so that their publish company would get more sales than those of the others. I wouldn't be surprised if the actual book names of these things were also selected by editors, or marketing folks from the book industry, just as the covers are.
Note that I do not subscribe to the claims of that subtitle. Nora's work has a lot to add to the Paleosphere, and I absolutely love her book, but in the big picture, it is removal of neolithic agents of health disease such as wheat, soy, PUFAs are the most important parts.
It's not necessarily the case that if you start from the SAD and then follow her way, the outcome would be better than if you start from SAD and follow Robb Wolf's, Loren Cordain's De Vany's, etc.
Art De Vany has The New Evolution Diet, soon to be renamed to the De Vany Diet, according to his announcements.
Mark Sisson has The Primal Blueprint series of books. I somehow don't believe that Grok, or any other caveman ever came across a blueprint, or despite the subtitle, that it would actually do any "gene reprogramming" (OK, so maybe it would enable or disable certain genes in your epigenome via methylation, but it certainly would not modify your actual DNA.)
Loren Cordain and Robb Wolf have stuck to the original "Paleo" term.
I think Robb Wolf's "The Original Human Diet" subtitle is likely the least inaccurate, and we do know based on coprolites, bones and other archeological finds what the original human diet was, but even so, it's not 100% accurate as there's no woolly mammoth steaks I can buy, nor any aurochs.
I'm sure Cordain would not have chosen "Lose Weight and Get Healthy by Eating the Foods You Were Designed to Eat", as we weren't "Designed" to eat specific foods, but rather we adapted to our environment of 1M-100K years ago, and have failed to adapt to the SAD, hence the neolithic diseases we suffer from.
Paul Jaminet has "Perfect Health Diet: Four Steps to Renewed Health, Youthful Vitality, and Long Life" -- I'm sure that there can be more, or less than four steps, and that one could attain "Renewed Health, Youthful Vitality, and Long Life" through one of the other diets I've mentioned here.
Ancestral comes from "Ancestral Health Symposium" which has no tag line, and does not appear to have been infected by marketing weasels.
Chris Kresser has the "Personal Paleo Code", and you might have noticed his podcast change names several times, also indicating a need to find the right niche.
There are other brands, "Paleo 2.0", "The Quilt", Archevore, Lean Gains, "Protein Power", not all of which necessarily refer to a Paleo life style, but are of use to us who live this way, etc.
Another example: on Art De Vany's forums, I've seen people say that they're on the NED (New Evolution Diet), and then I see an almost identical question posted on Mark Sisson's MDA forums, sometimes posted by someone with a similar name, claim that they're on a Primal diet. So at least some of the readers of these books and users of these forums are somewhat aware of this and going along with it.
I think the one common use distinction (the same way that Kleenex and Xerox were once brand names but became generic) is that Primal would include dairy, where as Paleo does not.
But then, even Robb Wolf would tell you that "if dairy works for you, God Love you", and I'm sure that Mark Sisson would have zero objection to someone who has a diary allergy from reading his series of Primal Blueprint books and still calling what they do Primal while shunning dairy.
Note: I own most of the books I've mentioned, and love the authors and their works, and don't hold anything against them. Each has a lot to offer us. Do not take the above words to be derogatory in any way. However, as advertising attempts to modify our thoughts and behaviors, it is vital that we practice critical thinking before purchasing anything, or accepting any medical claims: especially those made by the medical, pharmacological, industrial agricultural, marketing/advertising, or political professions.
I highly recommend Positioning: The Battle for Your Mind which can educate you on at least some of these issues, and how these folks attempt to manipulate us.