Many people assume that the meaning is contained in the passage, and the listeners job is to get the meaning out of the message. The meaning is not in the passage, but is constructed by the listener. This happens because listeners vary. Different people have different knowledge and different ideas about the world. A person with more knowledge about something may understand more than a person with less knowledge.
Different people have different purposes for listening.
Some people may want all the details, and others may only want to get the general idea. And so they will get a different understanding. Different people have different interests. If something is interesting, people pay more attention and will understand more. So different listeners, who hear the same thing, may have different ideas about what he speaker means.