When using Transport for London's (TfL) Single Fare Finder to find costs for journeys where multiple possible routes exist, Alternative Fares are sometimes available. These are listed with a note such as "changing at Vauxhall" to indicate the route for which that fare is applicable. However, these notes do not appear to be completely accurate.
Taking a journey from Surbiton and Watford Junction, and excluding the Zone 1 fares, gives us two choices: the default fare (applied for the default route via Clapham Junction and then direct via Willesden Junction), and a cheaper fare which according to the annotation is valid when "Avoiding Zones 1 and 2 via New Malden, Richmond and Willesden Junction".
However, it's also possible (and usually quicker) to get from Surbiton to Richmond via Clapham Junction, which is in Zone 2. Based on the annotation given, the cheaper fare is not valid for that route - but I don't see how TfL could know that you've journeyed that way. Since the only pink validators at Clapham Junction are on the Overground platforms, which you wouldn't visit if travelling to Richmond, you'd next touch in at the pink validator at Richmond - as far as I can tell this would then charge the cheaper fare (as long as you touch in again on the pink validator at Willesden Junction).
Also, the standard fare between Surbiton and Richmond doesn't have any zonal restrictions, so it seems odd that a journey incorporating that leg would add restrictions which only affect that leg.
Is the annotation wrong, and travelling to Richmond via Clapham is allowed, or is the fare really not valid even though there's no way for TfL to know your route between Surbiton and Richmond?