Why does Pip feel the need to lie about Miss Havisham when he is questioned about her by Mrs.Joe and Mr.Pumblechook? Why is he confident Mr.Pumblechook will not correct his story?

Mrs. Joe of course wants all the juicy details about Miss. H. Pip doesn't want to tell her because he doesn’t think that anyone would believe his account of the old lady in an old wedding dress, but he also doesn’t really want to Humiliate Miss. Havisham publicly. Actually Pip spins fantastic tales about Miss. H to Mr.Pumblechook who also wanted all the details about Miss. H.