If you went to any retailer that sells season tickets and asked for a season ticket from Reading to Coulsdon South, then you should be offered, as the cheapest option, a Reading to London Zones 5-6 season ticket. You should also be given the option of extending that season ticket, at increasing prices, to include Zones 4-6, 3-6, 2-6, and 1-6. I don't think anyone could say that, if you asked for a season ticket from Reading to Coulsdon South that, if you are then given a season ticket to London Zones 5-6, it wouldn't be valid along the shortest route from Reading to Coulsdon South, or a route within 3 miles thereof.
Why, therefore, would anyone legitimately try to suggest that buying a season ticket that is additionally valid in Zones 1-4 would mean that that route is no longer valid? Obviously there are cases where this happens, but I think anyone can see that is an entirely untenable position upon analysis.
Now, of course, there will always be those who disagree, but I struggle to see that this is any kind of anomaly or anything untoward - you are using the ticket in exactly the way you are supposed, and permitted, to use it. To me, the idea of London Zones 1-6 meaning that you can choose your destination (for the purposes of Condition 13) as being any station within London Zones 1-6, is no different to how you can choose any station within any other kind of station group, e.g. Manchester Stations, Birmingham Stations and so on.