Despite all the humans, English language, and proper British accents, it's important to remember that the Star Wars universe isn't our own. Things are different there they have landspeeders, they order around protocol droids, and their language although sounding similar has words that we think we know, but really have no clue. These are 7 terms from Star Wars that don't mean what you think they mean.
7. "Sith" is a species.
The Sith are the bad-guy Force people. Everyone knows that! They're the anti-Jedi, lightning shooting, Darth-named weirdos who refuse to use anything other than red in their lightsabers. Except well, they didn't start off as "the group of Dark Side Force-guys." They started off as a species unto themselves.
Basically, they were red-skinned humanoids with gross face-tentacles from the planet Korriban. They had a predisposition towards the Dark Side of the Force, but that wasn't what defined them. That happened once a group of human Dark Jedi happened upon the species and interbred with them for a couple thousand years. Eventually, the Sith species was pretty much bred out of existence, and Sith became synonymous with the evil, cloak-wearing dudes we know them as today.
Although, it probably would've been smart to try using any lightsaber color other than red if they wanted to not give away their allegiances immediately.