Lunch!

Traveling from Manchester to London is comfortable and quick. I hadn't travelled first class on this line before, and so was rather pleased to be given an early lunch on the house (train) as it were.
The sushi was delicate and light - and the chocolate was delicious!
Why is it that if you book ahead, sometimes the first class fare is not much more, yet at other times it is more than twice as much!? It even varies between trains in the middle of the same day.