Some more information for the possible debug, whenever (or if) anyone ever gets to it: It just occurred to me that IIRC the initial search results (i.e. page 1) always seems to return 120 threads. Maybe there's an artificial limit to the database search depth, which wouldn't be unreasonable to keep processor loading down, but maybe a relatively small number more optimal for smaller forums, in which case increasing it enough to be accurate for common searches might be a partial workaround. Covering the last few days might be adequate, since these were specifically available in the drop-down menu in the old vBulletin 3 version of SB.
And maybe as a human factors tweak to make this clearer, adding the words "at least" to the part of the search results which says "Result n to m of at least x" at the bottom of the page.