How does one know this? The Volkihar Vampires already in game are just powerful mages that are Vampires. How does one know their Vampiric Strain is the same as the Cyrodilic? We do know that we get a NEW Vampirism Disease in Skyrim.. Maybe this is the Volkihar Strain and most of the Vampires we see in Skyrim are ones that were infected? And the Special Volkihar ones are the original Vampires that infected them? Also, since it is a different disease, maybe it does have a secret that will lead to the Vampire Lord? And Bethesda is now deciding to release that secret? Also, the Volkihar Vampires from Dawnguard may just be more powerful versions than the ones we see in Skyrim now.. Maybe they are more powerful cause they are Ancients? We just gotta wait and see, now do we?
Movarth exists as he was taken by a member of the Order (Cyrodillic tribe), when attacked by either him or a Volkihar you end up with Sanguinaire Vampiris, this could portentially be just a different name for the disease, diseases can go by many names. Do either the Volkihar vampires with this strain that you speak of represent ingame law that's already established?
No unless they were of The Order or someone else, it would just be a bad cop out.
Don't get me wrong, I'm really looking forward to this fleshing out of the much needed 1 sided vampires in Oblivion and Skyrim, but does it pain people to read educated guesses and speculation concerning their origin or Bethesda's motivation behind said decision?
After all most of these ideas were created in the gamejam week and Bethesda mentioned that one way or another we'd end up with some of the results of that weekend in DLC. When people question they shouldn't be told to sit tight and wait for the final product before they can judge it, judging a movie by it's trailer seems to be the norm so why not videogames?
Hell even with leaked information we can't make even make well developed arguments?
Edit: Porphyric Hemophilia was the disease that was also caught in Morrowind, the clans varied widely by abilities and powers in that game so best guess is that the Vampires disease is not unique, just the Vampire that passes it on must also pass on some traits.