"Oh. Perhaps he is really crazy about you. Lucius doesn't seem the type to just go about inviting any friend to his Manor, does he?" From Bella's own interactions with him, she did not think he was very friendly at all, so his gesture to Narcissa surprised her. What was he playing at? Like she said, it was unlikely that their parents or Lucius's would arrange for them to be married.
Knowing his dislike of herself, Bella knew he would be positively thrilled to have her as a guest in his home. She wondered if Narcissa had already suggested her chaperone idea to him, and what his reaction was. She would have liked to be a fly on the wall during that conversation...