For somewhat obvious reasons, Google and the other search engines like to keep the true nature of how exactly they rank web pages in search results a secret; letting their closely guarded secrets out would lead to search results being so manipulated by companies wishing to rank highly as to render most results irrelevant. So, not everything that goes on behind Google's curtain is fully understood, and that isn't necessarily a bad thing, but fortunately there is enough knowledge available to help give sites a gentle leg-up advantage over the competition who maybe care a little less about their ranking.