Memory isn’t a concrete thing. It’s pliable. Even if you experience an event directly, as soon as you tell the story, the original version of events is just slightly altered. You experience it all over again, this time with new feelings, new thoughts, which causes the story to shift a little every time you remember it. Most of the time, the flexible nature of memory isn’t much of a problem. But when a collision occurs, it’s kind of important to remember the details correctly.