Clearly he lied to her and told her that he would see her as soon as possible. It makes sense that a girl as in love as Marianne would be upset that her suitor is leaving town abruptly, even if she expects him back in a few weeks. The real mystery is here:

"Months!" cried Marianne, with strong surprise. "No -- nor many weeks."

Mrs. Dashwood was sorry for what she had said; but it gave Elinor pleasure, as it produced a reply from Marianne so expressive of confidence in Willoughby and knowledge of his intentions.

Since Willoughby told Elinor and Mrs. Dashwood that he would not be back for a year, then why is Elinor pleased to hear that Marianne expects to see him so much sooner? She now knows that Willoughby lied either to her and her mother or to Marianne. And it is much more likely that he lied to Marianne, for why would he tell them what they *didn't* want to hear if it was a lie?

She should be upset that Marianne feels so confident about the intentions of a liar.

] Regarding that last meeting between Willoughby and Marianne at Barton Cottage in chapter 15: did anyone else get the impression that Willoughby told Marianne a different story than the one he told the rest of the Dashwoods?

] If he told Marianne that he had "no idea of returning into Devonshire immediately", why would she expect him, in the next chapter, before "many weeks"?

] But if he did say something different, what could it be? Whatever it was, it simultaneously threw her into great despair, yet had her expecting him back shortly. Any ideas?