Seems to me that the search parameters are sticky for some indeterminate period of time.
In other words, if Futile Gesture does that search first thing in the morning, then posts a few messages over the course of the day, and then does the same search at the end of the day, the listed threads will be exactly the same and in the same order as returned in the first search. Oddly, threads with new posts will be boldfaced, but they won’t be sorted by most recent post as they should be.
However, you can override this (sort of) by changing the date parameter. For Futile Gesture’s search, if you set it to “one week” in the morning, and then “two weeks” in the afternoon, you get a freshly updated result list.
Sometimes this works overnight, and sometimes it doesn’t.
I don’t know if this is a bug in the software, or a feature (i.e., a search is temporarily saved to make results come up faster, even though on a fast-moving board like this they wind up being inaccurate).