To keep the secret of her defection from their master, Antonin spun an elaborate lie about her traveling. The Dark Lord's mind had become so rotten and diseased that he couldn't always remember what he was told. Even repeating the same lie over and over again worked at times. Fearing that he might have already been told that she was back in Hogsmeade, Antonin didn't want to risk lying. While he knew that no other Death Eaters had access to their master, he wasn't sure what the house-elves who served him were telling him. Being honest was really the only option at that point. With Oliver still within the castle walls, they couldn't risk him being used to punish his parents.