port -f upgrade <port> rebuilds already installed stuff

I recall that using port -f upgrade outdated would do this (reinstall stuff that was not outdated) but that, until today, performing the same command on an individual port would only install what was out-of-date. Today, however, when I upgrade the few ports that were outdated, lots of stuff was rebuilt, some of it for every port I upgraded. What changed to cause this? I'm pretty sure this is new.