The argument the 'railway' would use is that if you planned to travel one leg of a return journey in the peak you would be sold/have to buy an Anytime return fare.
Excessing an Off Peak return to Anytime return is therefore the 'correct' excess in their eyes, if you originally planned to travel off peak both ways, but then decided to travel peak on one leg.
Whether its unfair is debateable, agreed. Another argument could be to abolish Return fares and only have Singles, priced appropriately. That would simplify Excess Fares.
The whole differentials between single and return fares, and peak and off peak ... and how they vary ... makes for a need to study whether it's best to buy two singles or a return in the first place.
Melksham (where else?
to ) ...
Paddington
Anytime return 193.80, single 96.90
Off Peak return 81.70, single 48.20
Super Off Peak return 61.30, single 35.70
Manchester
Anytime return 213.30, single 106.70
Off peak return 101.60, single 100.60
Plymouth
Anytime return 90.70, single 87.60
Off Peak return 75.70, single 48.50
Carlisle
Anytime return 287.20, single 143.60
Off Peak return 160.60, single 118.00
It's an inconsistent mess ... also noting things like off peak to Carlisle is "must not start before 04:15 on Mondays to Fridays" which - for the northbound journey at least - does not apply to any trains at all as the first train in our still-sparse service doesn't leave until 05:33 ...