If no zone 1 avoiding fare is listed by the fare finder then one won’t be charged no matter what you do.
The fares listed are the ones that available. If only a single fare is listed between two stations then that’s the one that will always be charged.
It’s a common misunderstanding that the traveller will be charged depending on the zones they actually use. They won’t. They’ll be charged one of the listed fares if their tapping pattern matches the requirements of that fare otherwise they’ll be charged the default fare for the journey.
As to why no fare is listed – that’s on TfL to explain. If I had to guess then I’d suggest that the journey to avoid zone 1 was deemed unreasonably complex for the normal traveller so not worth pricing differently (or used so much rail capacity through all the extra connections etc that it justified the price charged). I’m not wholly serious about the second suggestion, but a case could be made to support it.
Certainly, if such a journey existed in the database, it surely wouldn’t be the one you used as the SWR direct connection from Vauxhall to Clapham Junction is much simpler.
