Because query_see_board isn't invoked there. The presumption is that higher up the chain where the actual searching is done, query_see_board was already applied, and ordinarily that'd be fine. But here is not using any of the stock code, it's using Sphinx and the index just happened to be slightly behind (i.e. what was in the index had a different board id to what the actual DB had, but it was only matching up by post ids)