Following a tip off from a forum user I’ve discovered a number of anomalies where the fare from Epsom to stations in zones 4-6 is cheaper that the fare from Ewell (East or West as appropriate) to the same station. In part this comes down to different people setting the fares; South Western Railway/Southern are responsible for fares from Epsom whereas the fares within zones 1-6 are set by the Rail Delivery Group on behalf of all the train companies.
The big difference is for Worcester Park (4) where the fares from Epsom (£3.40 peak, £2.70 off-peak) are both 40p cheaper than those from Ewell West (£3.80, £3.10). The other stations, where the difference is 10p peak and 30p off-peak are:
Banstead (6 via 5), Belmont (5), Carshalton (5), Carshalton Beeches (5), Cheam (5), Epsom Downs (6 via 5), Stoneleigh (5), Sutton (5), Waddon (5), Wallington (5), West Croydon (5) and West Sutton (5).
If you live midway between Ewell and Epsom it might be worthwhile going the wrong way to Epsom for a day out in Sutton or Croydon.