Someone has an unusual talent. The twist is, they don't know it's a talent; it comes so naturally to them that they just assume that everyone else can do it too. This can also apply to...rather more physical attributes. Generally, characters like this are either a Cloudcuckoolander, Captain Oblivious, a Fish Out of Water, or very young and inexperienced. Depending on the feat, this can also be an Achievement in Ignorance. Truth in Television - you can only experience your own mind, so it makes sense to assume that other's minds work the same way unless proven otherwise.