Just because people are corporeal born to this world doesn't mean that they understand everything about it, similarly just because somebody's died doesn't mean that they understand everything about the incorporeal world.
人が肉体を持ち、地上に生まれたからといって、地上のことを全て理解するわけではないのと同様に、死んだからといって霊的世界を全て理解しているわけでもないのです。