Why because I've driven Diesels now for years and none of them driving hard have gone as low mid thirties and none certainly much below mid 40s. You examples were where used to try and wind you case by giving the best of your Golf and the worst of a Diesel not the average of both. The modern Diesels just get better and better economy with the add bonus of some performance.
I also know someone who switched from MkV Golf GTi to a Diesel because the GTi cost way too much to run.

