Google has been personalizing their search results to some degree for a while now. That means that your 10 site listings for a certain search might be different for another person who searches using the same keywords.
Though the personalized results haven’t brought radically different listings into play, they do impact who gets shown when and to whom. That means you’re not guaranteed to be #4 or #5 for everybody for your all important keyphrases. So what can you do?
Find out what your strategy needs to be in this article: