It's some balance between activity and thread saves. The list, as I understand it, is made up of the top ten most saved threads of that forum which are still currently active (unarchived), organized chronologically by time of most recent post. But, when a thread gets locked (for example, Clockman) I believe it can fall off the list without being usurped in saves simply due to its lack of activity because of it being locked. Does that make sense? I hope so.