Getting back to the original question: the cost of a season is based on the holder making one return journey a day for a set number of days a year. It is therefore not unreasonable that delay-repay should use the same calculation to determine the daily fare paid. If the holder makes a second journey in one day, then that second journey is effectively free. The delay-repay calculation is based on a percentage of the fare paid. And a percentage of nothing, is nothing. So I would argue that allowing you to make any claim at all on the second journey is being generous.
However, it would be very hard to administer such a cap. Suppose you were delayed on the outward leg of the first journey, and the return leg of the second journey, how would they know you had made two returns. So setting a cap based on the daily fare seems a reasonable compromise, and is more generous in that you can claim for delays on the second journey if not delayed on the first.