Why doesn't Pip tell the truth about Miss Havisham in Great Expectations?
In my opinion, Pip has realized an advantage he has over the adults in his life and exercises his right to explore that advantage. See, none of the adults know the real truth about who Miss Havisham is, but they have vague fancies about what must go on in her home. So, when Pip talks about the great dogs, and the flags, and the coach in the house, and eating on the gold plates he has them all going.
Another purpose this served for Pip is that he discovered that although Pumblechook claimed to know much about Havisham, Pumblechook really doesn't have any clue.