Looking through ATMB I came across several threads asking about the #straightdope (not affiliated with here, gotta say it) but they are all very old but show today’s date as the last post: http://www.cwcm.net/atmb.jpg
For example “How to participate in SDMB-related chat?” the OP started back on 12/27/00 and finished with a post by Gunslinger on the same date but the last post, according to the forum page was today (11/17/01) at 6:53pm.
Earlier today they all showed up as having been bumped by somebody. I presume a mod or admin took an action like removing the poster or their posts. Which suggests that threads have an explicit “last post” time on them in the DB, rather than the “last post” date corresponding to the last actual message in the thread, or the message is still there, and just not displayed when you read the thread. Whichever, whatever action was taken to remove the annoying “bump” messages didn’t adjust the date.