I've been using MacPorts for quite some time now. And ever since then I noticed that MacPorts keeps old ports floating around. Even a

port -u upgrade

doesn't help because as of 1.7.1 it only uninstalls the previous active port but not the remaining inactive ports. In order to get rid of all inactive ports you have to perfrom a

port uninstall inactive

and voilà, all your inactive ports are gone :)

In the upcoming version 1.8

port -u uninstall

will behave as expected according to this changeset.