In Great Expectations, why does Mrs. Havisham fail to correct Pip when he thanks her for his fortune?

After Pip takes leave of Joe and Biddy at the forge, he
goes to town and the tailor shop where Mr. Trabb has made clothes for Pip that will be
appropriate for his new station in life as he soon moves to London in order to be a
gentleman.  However, before his departure, Pip visits Miss Havisham; at the gate he met
by Sarah Pocket, one of the "toadies," who "positively reeled back when she saw me so
changed," Pip narrates.  As she escorts Pip us the stairs, she stares at him the entire
way.

When Pip enters, Miss Havisham orders Sarah Pocket to
remain in the room; she asks Pip his reason for visiting.  Pip cautiously tells her that
he is on his way to London and wishes to take his leave of her.  Then, Miss Havisham,
who seems to be enjoying herself, makes "her crutch stick play" around Pip as though she
were a fairy godmother, "bestowing the finishing
gift."

"I
have come into such good fortune since I saw you last, Miss Havisham," I murmured.  "And
I am so grateful for it, Miss Havisham!"

"Aye, aye!" said
she, looking at the envious Sarah with delight.  "I have
seen Mr. Jaggers.  I have heard about it
Pip...."

Questioning Pip
further as though she knows the answers already--"And you are adopted by a rich
person?"  "And Mr. Jaggers is made your guardian?"--Miss Havisham gloats on her
questions and the answers as she keenly watches Sarah Pocket's "jealous
dismay." And, as Pip leaves, Miss Havisham looks at Sarah
Pocket "with triumph in her weird eyes." When Pip leaves,
Mrs. Pocket continues to stare at him and does not say
good-bye.

Clearly, Miss Havisham delights in misleading
Sarah Pocket, who just waits for Miss Havisham to die, into believing that part of Miss
Havisham's fortune is being spent on Pip, causing the "toady" to becomes envious
of Pip's procurement of money from the old woman when she, a relative by way of Matthew
Pocket, her husband, has not seen the first shilling.  Also, believing Miss Havisham is
Pip's benefactor dismays Sarah Pocket even further as she calculates that whatever
inheritance Matthew may receive will now be greatly reduced.