Hi Chris,
You probably won’t be surprised to know that several issues are combining here to make the system so complex.
First of all, retail systems understand a travelcard to originate at a station and end at zones 1-6. Beyond zones 1-6 you get out boundary travelcards which are valid for one return journey from the origin to the zone 6 boundary and unlimited travel within zones 1-6. From the point of view of rail services an out boundary travelcard is finished on exit at the origin station because it’s not valid the other way round (zones – origin – zones). If you want to do the other way round you need a boundary zone to origin return to use with a standard zones 1-6 travelcard. It should also be noted that a travelcard is still valid to use in zones 1-6 until 0430 the next morning, but you’ll need to pay separately (or use a TfL bus) to get back to the zones. So, when searching for fares between two stations, if the origin is outside zones 1-6 and the destination is inside you will get travelcards quoted, but not the other way around.
Next you get the problem of zones 7-9. This is originally a TfL concept which has been kludged onto the National Rail systems in a sub-optimal way. Any day travelcard from a zones 7-9 origin uses a special route code which says (also available at) AAA zones 7-9. This expands the out boundary validity to include unlimited travel in the extra zones. It’s still classed as an out boundary travelcard, hence the suggestion that you’ll only get one return from Amersham. To further complicate things there are out boundary travelcards from Watford Junction, Broxbourne, Rye House, St Margarets (Herts), Ware and Shenfield which also include unlimited travel in zones 1-9. They only allow one return journey to the zone 9 boundary, although some staff may believe that it’s the zone 6 boundary.
Then you get the Kent problem. Swanley is now pretty much standard because the old Swanley to zones 1-6 travelcard has been removed from the system. Initially it was slightly cheaper to get some versions of the out boundary travelcard to zones 1-6 which was obviously fine if you didn’t want any other parts of zones 7-9. It was also a regulated product which prevented it being removed at first.
Dartford has an even bigger issue. You can buy tickets, including travelcards, which are valid via Gravesend and onto HS1. This means that there are two versions of every ticket, one with a route code saying “Plus High Speed” and the other saying “Not valid on HS1”. This means that the “AAA zones 7-9” route code cannot be used. So whilst a travelcard which includes zone 8 is absolutely valid at Dartford, if Dartford is the origin station then it is not valid at any other zones 7-9 stations. I’m fairly confident that the retail staff at Dartford would sell you an appropriate ticket if you ask, but this is definitely an issue which ought to be addressed somehow.
I hope this clarifies things, but do ask a followup if you want.