This might clear the air on a few things....
From All Access...
Mediabase Institutes New Recurrent Rules
Effective this week, MEDIABASE has modified its recurrent rules. On the Top 40, Rhythmic, Urban, Active Rock, and Alternative charts, downtrending songs below #15 are now removed after 20 weeks. On the Hot AC and Triple A chart, downtrending songs below #10 are now removed after 26 weeks, while downtrending songs below #15 are still removed after 20 weeks.
http://www.allaccess.com/
This definitely changes things! The #11 - 20 range gets clogged with months-old hits on the way down a lot so hopefully this will get rid of some of that.