From the beginning of Chapter 13:

Mrs. Bennet's eyes sparkled. "A gentleman and a stranger! It is Mr. Bingley, I am sure."

I don't understand this. How could Mrs. Bennet consider Mr. Bingley to be a stranger at this point of the story? He's met with and spoken to Mr. Bennet, Mrs. Bennet, Jane, Elizabeth, and Lydia. He's visited Longbourn. He's taken care of Jane and hosted Elizabeth. Mrs. Bennet, Kitty and Lydia have called on him at Netherfield. Yet he's still a stranger?