I the The Two Towers, in the Chapter "The Departure of Boromir" Legolas, Gimli and Aragorn are examining the remains of the orc and discussing the symbols (Saruman's) on them.

'S is for Sauron,' said Ginli. 'That is easy to read.'
'Nay!' said Legolas. 'Sauron does not use the Elf-runes.'
'Neither does he use his right name, nor permit it to be spelt or
spoken,' said Aragorn.

Yet, in "The Black Gate Opens" the Messenger from Mordor is named The Mouth of Sauron and he refers to him by that name many times. Sure it's a trivial little point, but does anyone know if this apparent contradiction has been explained?