Comment 9 for bug 706774

aptitude has a more powerful dependency resolver, if for any reason apt-get can't figure out what to do, then it will fallback to aptitude. This will happen if the revert list sent to apt-get is not 100% correct and complete.

I am not sure if the fallback ever happens now, it certainly shouldnt on simple ppa's but on complex ones that ship lots of core libs (like gnome3-staging) it may. A good test case would be to try pp-purge gnome3-staging on 15.10