[Spoilers Main] Why is Brienne always styled as "of Tarth"?

Every other noble man and woman in the series is referred ti on the modern style "given name, surname". Cersei Lannister, Ned Stark, etc. Even bastards follow the formula but with a different surname. So, why isn't Brienne referred to as Brienne Tarth, but rather Brienne of Tarth?