I’d agree with Feathers. Fares on late adopting NR services within zones 1-6 are set by representatives of the TOCs concerned under the umbrella of ATOC (now RDG). This group will include SWR, but I don’t think the people setting fares from Epsom will particularly have thought about the fares within the zones.
You could write to SWR and ask them if they have any comment.