The problem with the junk filter isn't a problem with the junk filter, it's a problem with the viruses. The viruses automatically rename themselves to whatever you search for.
For example a "south park christmas" search will turn up a virus called "south park christmas", and if you mark that as junk then do a search for basically anything else, like "family guy buttsex" a file by that name will turn up and may or may not be marked as junk also, depending on whether the virus has modified a byte randomly or not, causing it to be the exact same file with the same function but having one more 0 or 1 and one less 0 or 1.
Perhaps someone can explain this more thoroughly as I'm tired...