Why does Stevens call Mrs. Benn by her maiden name?

In the section titled "Day Two", Stevens has a rough time sleeping and awakes in the dark. In these quiet moments, he finds himself going over passages from Miss Kenton's letter. He says that Miss Kenton should be called Mrs. Benn, since she has now been married for twenty years. But because he knew her only during her maiden years, he continues calling her Miss Kenton.