Why does Pip believe Miss Havisham sent Jaggers?

During Mr. Jaggers' first momentous visit with Pip, Pip and Joe naively assume that Miss Havisham is Pip's new mysterious benefactress because Mr. Jaggers is Miss Havisham's lawyer. When Pip previously visited Miss Havisham throughout his boyhood, he had seen Jaggers at her home and discovered that he was the eccentric woman's attorney.

Of course, this assumption plays into Dickens' interwoven plot of characters and events since Pip, who is a poor judge of character, never considers that someone else might be sponsoring him and that person just happened to choose Jaggers to handle his/her funds.