Further to that, if you ask for the Cheapest ticket to London, you'll be sold a Super Off-Peak Ticket to Paddington. They won't offer the via Salisbury option unless you specifically ask for it!
Yep. I've asked a similar question of staff at Bristol Temple Meads. I knew what ticket I was after but thought I'd see what was offered when asking, "Cheapest day return fare to London please". I, too was offered a Super Off Peak Return, route 'Any Permitted'.
This goes against the impartial retailing rules, but I fully understand why I wasn't offered the cheaper route. It's unlikely that the incorrect fare is being offered deliberately. More a case that the fares structure is so complex that even the best trained staff will make mistakes.
Of course I know that it's even cheaper to ask for an origin station on the Severn Beach Line, or (as I recently learned) Bristol Parkway or Yate, when wishing to travel to London via Salisbury. This is true of all origin stations from the Bristol area, Bath, Trowbridge, Westbury, Salisbury and Andover and all shacks in between