My answer is usually along these lines: "Your site is competing for placement with many other professionally-built sites. If I'm doing my job properly, and so are the other web developers, none of us is or should be entitled to top placement. Also, placement depends heavily on factors that are solely in your control as the site owner, such as the quality of the content."
But when web devs say this, clients sometimes think we're just trying to shirk responsibility. So I would be really happy if I could point them to an official statement from Gogole that conveys roughly the same message.