Yeah, that's one thing that bothers me a lot. I guess Lucas thought "Huh, 'Lord Maul' sounds kind of stupid," and so made Darth into the new 'lord'. Which is stupid.
You can tell this wasn't Lucas' intention from the start because Obi-Wan calls Vader 'Darth' in Star Wars. If Darth was supposed to be a title from the beginning, why didn't Obi-Wan also refer to Vader as 'Lord'? It would make about as much sense.