It seems a bit silly of them though not to provide a breakdown of the journeys they took into account when drawing up the settlement. If GTR or indeed any other train company later discovers some other journeys before this date where you hadn't paid the correct fare, you can now say they were all included within this settlement and no train company can do anything more. If they had instead provided a list of the specific journeys, then if others were discovered later, the railway could come back and ask for more. So it's a surprising example of the railway not acting in its own best interest.