Miss Bingley... is not such a simpleton. Could she have seen half as much love in Mr. Darcy for herself, she would have ordered her wedding-clothes. But the case is this: -- We are not rich enough or grand enough for them; and she is the more anxious to get Miss Darcy for her brother, from the notion that when there has been one intermarriage, she may have less trouble in achieving a second; in which there is certainly some ingenuity

Elizabeth clearly sees the motivations behind Miss Bingley's farewell letter. Her reasoning is very sound given what she has already witnessed in Caroline's behavior.

*******

However, two events after the letter's arrival give doubt to her ability to truly read a character. Her first was the shock was when Charlotte accepted Mr. Collins proposal. Charlotte did not act as Elizabeth expected there, who assumed that Charlotte would not think of marrying such a stupid man.

Then it is Bingley who is not living up to expectations.

The idea of his returning no more Elizabeth treated with the utmost contempt. It appeared to her merely the suggestion of Caroline's interested wishes, and she could not for a moment suppose that those wishes, however openly or artfully spoken, could influence a young man so totally independent of every one.