Because according to this paragraph from wikipedia....

In British law, a commoner is someone who is neither the Sovereign nor a peer. Therefore, any member of the Royal Family who is not a peer, such as Prince Harry of Wales or Anne, Princess Royal, is (technically) a commoner, as is any member of a peer's family, including someone who holds only a courtesy title, such as the Earl of Arundel and Surrey (eldest son of the Duke of Norfolk) or Lady Victoria Hervey (a daughter of the 6th Marquess of Bristol).
http://en.wikipedia.org/wiki/Commoners_in_the_United_Kingdom