SEO is not an exact science. You can read this article from Forbes Magazine Why No Reputable SEO Companies Offer Guaranteed Results. SEO is an algorithm that has been designed by the search companies to tell the search engine how important each site its and if it's relevant to what the searcher is looking for. Each search engine company has their own criteria and secret algorithm. They also change and update these often. So a site that may have ranked high for it's keyword last month doesn't this month due to an algorithm change. This is why you have to stay on top of the SEO for your website. As the algorithms change, so will your ranking. It doesn't happen overnight and is a gradual process. There are a lot of factors involved. The search companies do this so the search engines are not over run with crappy websites that offer nothing only spam.
If at any time you are not happy with your results you can just cancel your monthly service subscription. It's that easy, no questions asked. We do however not continue to optimize your site after the fact and your site will return to it's original state before we optimized it.
Disclaimer - We do not guarantee in the sense of the word that we will put you on the top of the search engine. If we told you that there was a guarantee we would be lying to our customers. However we do our very best to in every practical and ethical way possible to put your site in the range of the top 3 pages of the search engines.