See the LATEST ACTIVITY in the right bar. Currently it shows misleading information.
See Debate Society. Latest activity shows Jun 30 '10 at 4:02, but the proposal has more recent activity. See recent followers (Nov 12 '10 at 0:59), comments (Sep 5 '10 at 19:40). I can't see when the last vote was cast but I'm pretty sure it wasn't 6 months ago. That proposal is not a single example, there are hundreds more. Another one is The Practicing Musician.
This report originated from What's the time necessary for a proposal get deleted on Area 51? See comments on Robert's answer, especially the last one.
Is it a bug or by-design? For me it's a bug because it provides misleading information.