I think one can also, however, interpret Lizzie's responses as not only finding the most potentially humorous approach to what Darcy has said, but also firmly retaining her belief that he considers her merely "tolerable," and taking any statement of his as an attempt to disparage her. I think Darcy is in many of these scenes trying to flirt (subtly, clumsily) with Elizabeth & she is having none of it. That's why he accuses her of willful misunderstanding.