The reason for that is because how Google actually works is complicated. How does it manage to look through millions of web pages and produce a credible result for your query in a couple of seconds? It uses more than 200 signals to look at the pages in its index and compare it with the search term to work out which pages are the best ones to recommend.
Matt Cutts, the face of Google’s quality team, has presented a nicely designed video explaining how Google works. It simplifies the whole process of how it selects which pages to display in results and in what order.