Thanks Mike. If you’ve not yet had the answer, here is some more evidence as it seems that there is still misleading fares information being published on these routes.
For Richmond (rail) to Slough off peak, nationalrail.co.uk advertises a contactless fare of £9.70.
TFL / Oysterfares records a fare of £4.50.
Having made the journey, TFL split it at Windsor and charged £8.70 (£6.80 + £1.90)