The point of view here is third-person limited. This means that the narrator is not a participant in the events that take place, and they know the thoughts and feelings of only one character. In this case, that character is Monsieur Lantin. We learn, for example, that he "was unspeakably happy with [his wife]" and that she was so charming that he discovered, after six years of marriage, that "he loved his wife even more than during the first days of their honeymoon." We never hear of the thoughts and feelings of Madame Lantin, and, therefore, we are in great shock right alongside her widower when we learn that what seemed to be "imitation jewelry" (indeed, what her husband had always believed to be imitation jewelry) is, in fact, real and quite valuable. We do not expect this revelation, because he does not expect this revelation. Were we to know Madame Lantin's thoughts and feelings all this time, then there would be no mystery, no truth that must come out. However, because of the narrator's perspective, the story acquires a mystery that it would not otherwise have.