Bella called Voldemort "Master" during the Battle of the Department of Mysteries, particularly when she was pleading with him when he was angry, so my head-canon for this story is that she was deliberately reminding him of her submission, and his subsequent tenderness here. And I did like the dynamic she had with Sirius here, especially Sirius taunting her that she wasn't a warrior, only for her to kill him the next day, and the tension between Sirius' attempts to slut-shame her and her complete refusal to be shamed.