Asajj Ventress is a SITH!!!

Remember that the Jedi had no idea who the second Sith was. They knew Dooku was, but they didn't know if he was the master or apprentice. It would have been natural to assume Ventress was the apprentice up until the time Savage took her place. But she was never a Sith. She was never given the Darth title. Sidious expressly forbade Dooku to treat her as a Sith or an apprentice. She was merely an assassin who worked for him.

Savage would be no different if he were the Savage under Dooku's employ, but when he was with Maul they were calling themselves Sith, and they saw themselves as having their own rule of two, because they wanted to eliminate Sidious and Dooku. That violates the rule of two with other Sith already in operation, but it's not the first time a rival pair had arisen to the two who were in the line from Darth Bane, and there were a number of times in the Legends canon when a third Sith was in training while the two were still both alive, e.g. Darth Maul was actually the third Sith at the time of Episode 1, since Darth Plagueis was still alive at that time. Plagueis was even still alive when Sidious took on Dooku.