Very good question. If you lie, you have to remember which lie you told to who. If you tell the truth, since there can only be one truth, you don't have to remember what you told to who. If they ask you again, and you tell them the truth, the story will be the same.
Law enforcement uses that all the time. They lock you into a story on tape during the first interview. Two weeks or a year later, they interview you again. If your story changes a little bit, you lied. If you told the truth, there's nothing to remember, tell them the truth again, it's never changed.