This is something to keep in mind. It has always been this way, is this way and will always be this way. Food for Thought by Dr. Kevin Nunley Search Engines Search engine databases change constantly. They get new listings, switch their standards, drop listings, and just plain lose parts of their database.
I would add that when they rank web sites for keyword searches, they hit and miss the targets all the time -- consistently being inconsistent. Regardless how sophisticated these search engines are, they are just machines. Their intelligence can only go so far.