The web is a dynamically shifting place and there are new websites added, new pages added, the content gets updated and as such Google takes into account these factors amongst others to continually assess websites and rank them on the SERP. This is the reason why you may see a website on the 1st position today, while it may slip down to the 6th position after some time. Google also keeps updating its own algorithm which affect the ranking of the websites. These updates are rolled out a few times each year and it therefore becomes important that SEO campaigns are undertaken on a continuous basis to maintain rankings and position.