Hi Michael,
This is the secondary usage of pink readers, to say you’ve only used TfL services instead of you’ve avoided zone 1. As with all journeys, a decision has been made on the liklihood that people will take any specific route. In the case of Denmark Hill I agree with TfL that you’re far more likely to stay on the Northern line to Clapham North and then use the Windrush line from Clapham High Street as it’s a shorter route and only one change of trains. In the case of the Forest Hill line they clearly reckon that Underground/Underground/Windrush is the most likely route, and the alternative via London Bridge can be identified by the interchange there, so they don’t mandate the pink readers at Whitechapel etc.
You may find they add Denmark Hill as a result of your query, but I wouldn’t be surprised if they decide it’s not worthwhile.