There are several different "forums" when you use the recent posts search option you get a mixture of all these forums.
If you stay within a particular forum you only get those threads.
If you post a thread in a tech forum, it will stay near the top of that forum until a reply is made then yes, it will fall down the page. It would rarely come off the first page in it's home forum (IE: 2005+ Talk)
However, in the "main" search page they do go down the page faster. That is just how the site is set up.
Most people use the main search page then go into each forum for more recent posts. The farther down in the respective forum the more activity the site has had

Just a matter of retraining where and how a forum is viewed
