Shoot... quoted me before I edited.
Tell you the truth, I think it probably has something to do with the quality of the search engine. Either it's too good of quality therefore considering too many parameters or it's junk and can't come up with anything when you give it more than one thing to consider while searching.
I belong to a few small car forums with no more than 5000 members and the search engine there just blows hni's away. No offense intended, it's just one of those matter of fact things.