Thanks for those links.
The first point to note is that fares on the line to Reading and associated branches were set before Project Oval. This means that the fares data sent to TfL does not come directly from the NR fares database. It looks like GWR have increased some of the off-peak fares but this hasn’t been reflected in the TfL database. This was probably at the September fares change where off-peak caps were all increased by GWR. I’m confident that the fare shown by the fare finder is what will be charged.
The Marlow issue is slightly different because the £22.00 fare is not tagged as being based on the contactless fare. I don’t know why the NR journey planner is not using the contactless fare when a fast train is being used. I’ll ask the question.