Twitter search was originally another company which was purchased by twitter latter on. It recorded and indexed tweets without any magic sauce, but in a way that other developers could as well. Thus, even if search.twitter doesn't feature a given tweet, any arbitrary API user could have it, and there is no way for twitter to remove those. Thus perfection is out of reach, and even if they did delete results from search, it would be more of an act of appeasement than an actual technical solution.