Great Expectation Question

In chapter 27, page 223, Joe is vistiting Pip and telling him what he has been doing in the time where Pip has been gone. Joe is telling Pip that Miss Havisham called him to go see her. As Joe is saying, "Next day, sir, ... having cleaned myself, I go and I see Miss A." Throughout the book we have only read that Miss Havisham is Miss Havisham not Miss A. or any other name. My question is, why does Joe refer to Miss Havisham as Miss A. when he is talking to Pip?