Interesting one! I think that's a bug in the way the ticket machine software is interpreting the validity information in the fares database. There is a field in the validity tables that specifies a day of the week that must pass before the return journey can be made. For these "Weekender" tickets, that is set to Friday. Fairly clear I think; Friday must pass before you can return.
But it looks like the ticket machine software has a bug. My guess is what it is doing is:
- Excluding the current day from the "day which must pass" calculation, so it thinks the Friday that must pass is next Friday
- Compounding this with a further bug where, rather than reasoning that the first day the return journey can be made is the following day (ie. next Saturday 19th November), calculating the offset between the Friday that satisfies the requirement and the date of the outward journey as 7 days, and simply adding that to the outward date to get the first valid return date
Needless to say, both of these are wrong.