How come some people with atrial fibrillation don't even know it?

Depends. Atrial fibrillation by itself does not produce symptoms usually, it is often the fast heart rate that may result from it that causes the symptoms of "fluttering" in the chest. Even with fast heart rates some people do not feel anything in their chest because not everyone is as sensitive to these things to the same degree. Some people, feel fluttering and it is not related to their heart at all.