Why does my search not show the latest references?

I’ve noticed that often when I search for my college (Bates College) I’ll get a hit for a recent post but a day or two later an identical search shows only older posts on an older thread. For instance, this thread

http://talk.collegeconfidential.com/sat-act-tests-test-preparation/2179383-this-student-scored-a-19-on-the-act-heres-where-she-got-accepted.html

is a featured discussion and has 3 different references to Bates, two of them from this afternoon. Earlier today this thread showed up when I searched the latest posts, which is how I found it, but the latest hit I currently get on a latest posts search is this thread, which last saw its last post on March 23rd.

http://talk.collegeconfidential.com/bates-college/2179029-bates-short-term-cancelled-should-any-refund-be-due.html

I’m sure it’s not particular to this one college, but what’s up with the algorithm? Why are we not able to see all posts? Is it because the search term isn’t in the title of the more recent thread?