[William Cloud Hicklin]

Or bonus Homerus dormitat. I suspect that T's thought when he wrote Aragorn's statement was that the epithet the Elves considered his "true" (i.e. their) name - Sauron/Gorthaur "the detestable" - was an insult that the Dark Lord naturally would have resented and never used, just like nobody I'm sure ever called the Duke of Normandy "William the Bastard" in his presence, or for that matter Grima ever called himself Wormtongue.

But then in the later chapter T simply forgot. For both simplicity and the requisite awesomeness, far better for the line to have been "I am the Mouth of the King of Kings" or "the Lord of Middle-earth" or even just "the Dark Lord."