Not that I agree with it, but if there's virtually any word in a post that matches something in another post, it gets merged into that older post. It may not be that people are adding to those threads, but instead that the threads are getting merged.

I can sort of understand it, it's not unusual for four or five threads on the same subject (or more) to be started in the same day around here, without the merges it would get way out of hand very fast. But I do wish they would purge some old stuff or dates to an archive so that there was not so much old stuff in the same thread as new stuff.