The Sith we see in KOTOR and the Sith we see in the movies function quite differently.
In the movies, there's only TWO. For all intents and purposes, they both can be "Dark Lord of the Sith" because there's no other Sith. I also think Palpatine considered himself "above" the title "Dark Lord" (particularily once he became the Emperor) which is why his apprentices were called that (especially Vader & Tyranus).