Should all be off peak anyway as paper tickets don't have evening restrictions...
This is what makes contactless more expensive, especially out of Paddington where paper tickets let you use stopping trains with an off peak ticket during the evening.
Single leg pricing only really works with an afternoon peak to match the morning one, the other option would be to charge much more for journeys in the morning peak. If they did that, there would be more anomalies compared with the fares within London.
It is somewhat inconsistent that TfL Rail services can be used with off-peak 'paper' tickets in the evening peak out of Paddington (and not, where GWR is the fare setter, for almost all GWR services), yet TfL's own fare structure on Contactless charges a peak fare to travel on the same TfL Rail services in the evening peak (obviously dependent on the first touch in).
I appreciate that travelling from the distant side of the Contactless area may mean that passengers could be passing through Paddington well after 8pm and need to be paying a peak fare, having touched in before 7pm. That is even worse if trying to connect to the branches. I guess the point is to try and break the through journey somehow.