Have you ever played the game where your sitting in a circle, and one person is given a piece of paper. That person reads the story on the piece of paper and then whispers the story in his or her own words to the next person, that person then whispers the same story in his or her own words to the next person. If you have more than 6 people, the story is virtually unrecognizable by the time it gets back to the same person.
I think posts work almost identically and it is because we choose the main idea of a given topic based on what we can relate too.
For instance, if a topic is about cars, and a respondant knows a lot about car engines, then they will find some way to relate the topic of cars to car engines and respond accordingly. The next person sees that the topic is about cars, and the discussion has moved to car engines. He or she doesn't know much about car engines, but knows a lot about small engine repair. etc...
Our worldview and areas of expertise, along with our experiences color our perceptions.
These in turn, direct our responses to different topics.
At least, that is my observation.