After Darth Bane only two Sith would remain at any given time. As it is said in Episode I, always two there are, a master and an apprentice bla bla bla...
In KOTOR however the title of Darth is used far before Darth Bane ever adapted this method.
BioWare had this to say when they commented on it specifically..
BioWare:BioWare's own explanation is actually contradicted in their own game...
We’re very careful to keep everything within Star Wars continuity. LucasArts also works with us and Lucasfilm Licensing to ensure that everything in the game ultimately makes sense within the Star Wars timeline. And of course, people will always have questions about how certain things are possible within our timeline, but rest assured that we have spent a great deal of time making sure that the story is as accurate as it is compelling.
With regard to the Darth issue, the title of Darth actually refers to Dark Lord of the Sith, which was a traditional title held by the most powerful members of the Sith. Thousands of years before Darth Bane created the new order of the Sith, the Sith were not dark Jedi, but were actually a civilization of people. The dark Jedi came to their planet after being banished by the Jedi and became intertwined with their culture, creating a cult around the dark side of the Force. The leaders of this cult took the traditional name of Dark Lord of the Sith, and some were referred to with a shortened version...Darth. This all occurred well before the time period of the game.
Though some of the Dark Lords in the Star Wars timeline prior to the game didn’t use the actual title of Darth in their name, Malak does, for reasons that will become apparent within the story...
I'm curious to see if anyone else had thought of this before and what you guys think about it.