I think a part of this problem is just simply a matter of scale. As more and more information gets generated and stored on the internet, finding the information you exactly need in 3-5 words typed into a search engine at some point just ceases to become probable or even possible. So search engines have to predict what you want based upon the limited information you give it (and does this using others things such as how often something is visited, search patterns, trends, and/or related promotions), and you end up perceiving it as it telling you what you want.
Sadly giving a search engine too many words can stop it from even searching, probably because that would cause a scan or result in a very time and computer/resource intensive search. So it doesn't even bother searching and tells you it couldn't find anything.
