Pip doesn' want to let on that he didn't have a good time at Miss Havisham's. He also doesn't feel right about telling others about her personal life. Talking about how time has stopped in her house, that she still wears her wedding dress, and that she made him play cards with Estella (who very openly didn't like him) are all things that Pip feels he should keep to himself. He doesn't think its his place to share these things. He starts making up a story that he thinks they will believe, and Pumblechook's eager agreement shows his lack of knowledge about Miss Havisham and her house.