Norms are a product of differentiation. Social systems differentiate expected behavior (norms) from actual behavior. That is to say, norms serve as prediction schemata. The norm is the expected. Behavioral norms are expected even if they are not anticipated. For instance, even if we know a person well and we anticipate that he will lie to us, we nonetheless expect him to tell the truth; otherwise we could not engage in communication with that person at all.
The most controversial claim made by Luhmann is that society consists not of “actual people and relations between people,” but rather communication–and only communication. Society is a communication system, and there are a number of distinct subsystems that have evolved; yet they are all “communication systems” (Luhmann, Introduction to Systems Theory, 28).