Hi Alexander,
Beyond zones 1-9 there is no set rationale for setting fares or caps. They are both set by the operator responsible for the fares. In the case of Reading to London this is GWR. Because PAYG fares are all singles, peak fares are charged going away from London in the afternoon peak. This is so that commuters pay two peak singles (equivalent to an Anytime return). If all your travel is after 0930 then the off-peak cap will apply and will limit the effect of a peak single.
The routeing guide is not really about fares in the same way my tables are for zones 1-9. It’s concerned with defining permitted routes.
Until TfL work out how to handle discounts on contactless then sadly you may find paper tickets with a discount are cheaper than contactless fares.