Are the ticket machines journey planner based? If so, presumably the problem is the same as when buying tickets online (with a journey planner), as discussed previously
here and
here: The origin and destination you enter are only used to generate the itineraries, and once you select an itinerary the tickets you are offered are only matched to the selected itinerary, ignoring what you entered to start with.
It's a bit awkward for the ticket machine because it can't know whether a customer entering "London Terminals" as destination is looking for the cheapest way to London or a more expensive ticket with "London Terminals" printed on it, or indeed a ticket valid to any London Terminal station (which someone unfamiliar with rail ticketing might believe is a standard ticket type!). I guess the customer is expected, after having selected an itinerary, to understand that they have been offered all tickets valid for that specific itinerary, and need to look at the offerings carefully to see which ticket suits their needs.
I think journey planners should not allow entering as origin/destination technical terms like "London Terminals" or "London Thameslink" that have special meaning and can be printed on a ticket as origin/destination, to avoid the customer thinking that all tickets offered will have that specific origin/destination. National Rail Enquiries only allows you to search for "London (All Stations)" which is much less likely to be misunderstood as only generating "London Terminals" tickets.