Nick assures me that this is due to delayed indexing (every 15 minutes) on searches that are done via lucene. So: if something changes from closed/not-closed in that window, it may be considered (as far as the actual search is concerned) incorrectly. When we know the data we want to show, we then show the actual state, without re-applying the filter to the UI.
We could potentially investigate a post-filter scan, to remove any that don't actually match when it comes to the UI. We already do this for our tag-based searches (if they have been deleted etc while we weren't looking).