On reserve, you get leveled for whatever the trip is worth as flown. As a pilot with trip guarantee, you get paid the higher of what the trip was worth when assigned, vs flown. IE, if you bid for a trip that was worth 18 hrs, and changes turn it into a trip that would be worth less, you still get the pay you signed up for. On reserve, you do not hold trip guarantee, so you are leveled for whatever the trip is worth as flown.