It is true that they did not spend much time together but she is the princess of Alexandria, you can be sure that even if people do not know her personally they at least have seen or heard a bit about her, even though that is not always fair, also very often it is enough to just experience how a person is in a few moments. Just as Marcus noticed, what he has seen from her seemed different but one moment later a lot of her character that he already knew came through again. So the change was not that different, just a bit of something unexpected that she would have never done before or he at least would not have thought she would do or act like.