Rogues, probably.Ā Every society has its bad apples, even one that's fundamentally flawed in the first place (e.g. vampires).
Other vampires are still your enemy regardless if they are called Volkihar Vampires. The only true Volkihar Vampires that won't attack you are the ones in Castle Volkihar.
I don't know if you noticed this before, but when you block out the sun with Auriel's Bow, there comes a time where rougue vampires and gargoyles will attack out in the open...even you...even though you are also a vampire.
I know it seems strange that they would also be your enemy. Would have been better if all the vampires were not your enemy anymore if you're also a vampire, but yeah, I agree that this system is pretty flawed in that regard.
Remember the Bloodstone Chalice quest, two Volkihar Vampires from the court attack you at the end of the quest to take the credit for returnig the Chalice. It's just the way a lot of vampires are, trying to impress their Master Vampire to gain favor and power.
Plus a few of the side quests for the Volkihar vampires have you killing other vampires ("Culling the Beast" and "Protecting the Bloodline"). Apparently vampires just don't look out for each other. Everyone for himself?
Volkihar is basically a race of vampire, as stated inĀ Immortal Blood. It simply means that this vampire was infected by another from Skyrim and they happen to be gifted (even more so than regular vampires) in ice magic.Ā