Hi Roger,
I’m guessing that one journey was made during the afternoon peak, requiring the anytime single fare for that direction coupled with the off-peak single fare in the other.
The changes were introduced on December 3rd, so you just missed the change at the end of November. This is very much being driven by the Department for Transport. I’m not sure that they fully understood the implications for people travelling in the afternoon peak. They were supposed to happen at the same time as the contactless rollout, but that got delayed until the Spring.
When contactless does get rolled out I would expect the fares to be capped at the cheaper of (a) Sevenoaks to zones 1-6 travelcard and 2x extension fares to Bletchley, or (b) Bletchley to zones 1-6 travelcard and 2x extension fares to Sevenoaks. Unfortunately the DfT are being ultra secretive about exactly how the extension fares will be set, and TfL are complying with the instruction not to discuss this publicly. It is very frustrating.
Even worse for people like you is that concessions are not due to be implimented until phase 2 of project Oval, and that may be quite a while away given that phase 1 is now very late.