You may not force the upgrade of packages using
-f but you are using the
--force-config option which seems to enforce certain things as well. Added up with
-a which basically means "
do this with all installed packages" and you basically got your answer.
In the end my advice would be to stop using
portupgrade and instead use
ports-mgmt/portmaster. It provides the same functionality, maybe even better, with much less overhead (
portmaster is a shell script which fully utilizes the already present package management tools whereas
portupgrade fully depends on Ruby).