It's the nature of what tracks are, unfortunately. It's not a video, its a recording of all inputs. When you play the track, the sim actually flies the mission again, based on the recorded inputs. So when anything in the flight model changes, or probably in this case, a random generator decided that the sam you evaded hits this time, the track differs from what you did in the mission.
Well, the positive about this is, you can take control at any time and re-play the mission from that point on.
But I agree, a simple video replay additional to this would be very nice.