The travelcard covers his journey as far as Northfields (zone 3) and not
beyond. He then needs an extension which is valid from Northfields to
Heathrow, which has to be a zone 3-6 single. Your logic would work if
Northfields were a boundary station in both zones 3 and 4, but it isn't.

No, I think it's your logic, that is strange. With your logic if I have
ticket for zone 3, and separate ticket for zone 4, I cannot travel
between zones 3 and 4 without buying 3rd ticket for 3-4 combined.