Whether or not Ventress was ever Sith is... debatable. However, characters are snapshots. At one point, Ventress was a Jedi, but the Ventress we have is not a Jedi.
It's not undreasonable that the snapshot we have is of Ventress between Dooku's betrayal and the assault on Dathomir.
Also, tags are mechanical decision, not just lore. Ventress is mechanically made to integrate with the Nightsisters, but not the Sith. Asajj is already the best leader for a potent faction. Turn her into a bridge lead into one of the most powerful factions in the game and it could easily get out of hand.