Some of the posts in the thread are making a problem where none actually exists. The way the Oyster system works is that you need to:
- make sure you have enough Pay as you go credit to travel through the zones you intend to travel through which are not covered by any Travelcards you hold
- touch in and out at the very start and end of the journey
- touch at any gateline that you go through
- touch any pink reader you pass,
... then you will be charged the "right" amount for the journey.
The possible fares for a journey can be found at the
TfL Single Fare Finder (sadly it doesn't take into account Travelcards that cover part of the journey). Where there multiple routes that TfL deem reasonable and there are no intermediate gatelines to determine which way you went, the system will give you the benefit of doubt and charge the cheaper route. TfL know about this and it is allowed.
For the flows where this would cause a big revenue loss, TfL have installed pink readers to disambiguate the routes, but they don't want to install pink readers everywhere, so sometimes you can indeed go via additional zones without extra payment.