However, a lot of people will buy the cheapest ticket, and try to travel a more expensive (usually because it is quicker or more convenient) route, and then try to argue "The man on the platform said...".
We're not talking about 'a lot of people' nor 'what the man on the platform said' but one specific example from a specific booking given in writing (as it were) on a TOC booking website.
Contrary to Flamingo's apparently automatic assumption that every passenger must be up to no good, no subterfuge or deceit would have been entered into to obtain the tickets offered - just entering a journey requirement and time in the perfectly normal manner that would be used by any perfectly normal customer, not familiar with the byzantine compexities of the ticketing system, routeing guide and validity codes discussed at great length here, and which can apparently perplex even the most experienced members of railway staff.
Other similar discrepencies have been mentioned on this forum before, one by myself. Use it and stand your ground, I say, and if you have to later complain to a TOC about jobsworth attitudes like Flamingo's, so be it. Consumer legislation and 'fair contract' law will be on the side of the passenger every time.