In Chapter 9, why does Pip lie to Mrs. Joe and Uncle Pumblechook about his day at Miss Havisham's?

Pip's motivation to lie about his day is rooted partially
in the reality of knowing that Mrs. Joe and Uncle Pumblechook likely would not really
understand if he tried to explain the true situation.  However, knowing as we do Pip's
desire to better himself in terms of his social station, we can infer, and Pip later
confirms in a private conversation with Joe, that he hopes that others will reshape
their perceptions of him if he puts forth these amazing stories.  If people believe that
he, Pip, has witnessed the sumptuousness and grandeur of Miss Havisham's private inner
world, then doesn't he personally stand to become grander in their eyes as well? 
Ironically enough, Joe, although speaking in the language of a lowly commoner, shows
great wisdom as he points out to Pip that if he (Pip) can't improve his social station
through honest and sincere means, he surely won't be able to achieve such a thing
through dishonest means.