It seems the fares the journey planner website displays are limited by the other journeys listed in the same query. As far as I can tell, return-off-peak and day-return-off-peak are valid on slightly different times of day, but there's no way to ask (either an English language terms-and-conditions, or a lookup query) which trains a certain ticket is valid on, the only option is to try those routes and see if a ticket with the same name is available or not.
I know I shouldn't be surprised any more, but I always feel so incredibly wool-headed when someone is yelling at me that I urgently need "foo" but they refuse to tell me anything about what foo is or what it costs.