Doubling back is permitted as part of the shortest route. A route within 3 miles of the shortest route is also considered to be the shortest route.
Give it a go yourself. Enter Weston to Bedminster, via Temple Meads into NRE‡ and it'll tell you to buy a Weston to Bedminster single.
If only it was so simple
For the 06:00 ex-WSM this works, with an SDS of ^6.10
For the 07:08, 07:37, 08:41, the NRE
OJP▸ tells you that two tickets are required (total ^8.00)
For the 09:45 onwards, it's back to one ticket, the
CDS▸ at ^6.00
...Until 15:10, when it is two tickets again (^ 7.80)
....and a few more glitches until the 06:01 (Saturday) which is 2 tickets...
Why?