How can the title of the story "The Canterville Ghost" be justified?

The title of this story is justified because it really is a story about Sir Simon, the Canterville ghost. When the story begins, the reader thinks that this is a story about the Otis family and their move from America to an aristocratic mansion in England. As the story progresses, however, it becomes clear that this is, in fact, focused on the Canterville ghost himself. The narrator frequently mentions his haunting career, for example, and his numerous tactics to frighten the very skeptical Otis family. Much attention is also paid to the emotional state of the ghost, as his haunting ability is mocked by the Otis family.

This idea is further reinforced in Chapter Five when Virginia Otis and the ghost share an intimate conversation. The ghost reveals the truth about his marriage and confesses to Virginia that he is desperate to go to the Garden of Death where he can sleep in peace forever. The story is, therefore, transformed into a tale of the ghost's redemption, as well as the very unlikely friendship which he develops with Virginia.