I am in trouble for not explaining properly why rankings do change so often. A client has been requesting a manual — er, recommendation reports which details how their keywords are ranked. When she checked it, the results were off and asked for explanation.
I myself am not very convinced with my response. Anyway I had to explain. One of the reasons why this happens is that search engines have different data centers which accommodate the millions of searches taking place every single day. Another one is more dubious: search engines update their algorithms that any change in rules and patches will have affected the results.
With this experience, I feelt that keyword ranking is only good to show prospect clients how SEO improved their keyword ranking on search engine results. But doing it as baseline report when closing a deal is not the best option. Real results should come from traffic and sales and real return of investment. Keyword results are at the mercy of MSN, Google and Yahoo! which I have no control at and does not translate to sales.
Ultimately setting up a website, apart from telling the world that you have a new labrador pet, is to generate money.