Forum:KB:Sith Lords

"Despite the established anti-Jediism of the New Order, many in the Imperial government were aware that Emperor Palpatine and Darth Vader were Force users, but only a few knew that they were Sith Lords." (From the Galactic Empire article)

How did they not know Vader was a Sith Lord??? I mean wouldn't "Darth" Vader give it away. Also they always called Vader and Palpatine "my Lord"