Thanks Mike for the comprehensive explanation.
It does seem a raw deal on the suburban lines as TFL rinses non-Londoners with special conditions outside Zones 1-6:
• No discounts for railcards (Oyster not supported on Reading line)
• Fare increases above RPI while Zones 1-6 have a fare freeze or token rises
• Additional mid-year fare increases for Travelcards
• Journeys split and charged as two with traditional NR OSIs not supported by TFL
• Emasculated delay repay scheme with no payout if delay not the fault of TFL
• No Zonal ticket available on Heathrow Elizabeth Line branch