User blog comment:Noctosphere/Why is there still Leviathan in Purgatory?/@comment-1031724-20160504000540

Some of the Leviathans held on when Castiel returned the souls. So some Leviathans did return to Purgatory. Furthermore, two Leviathans were either bibbed or eaten by Dick, so maybe Dick killed more Leviathans this way when they disappointed or offended him, and when Leviathans die, they'll go to Purgatory