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. And that is ok, because these different ideas about the speaker's meaning may all be reasonable. The purpose of listening is to get a reasonable understanding of what the speaker said, not the 'correct' understanding. So what does all this theory mean for how we study listening.