Gone are the days when a site used to stay in a position for months even years. It is now been seen that the results have a lot of difference day after day and computer to computer. So, if your website shows up at No.1 position do not be surprised if it shows in the second page a day after.
The results are also not location based. As there are huge differences in results on two computers in a same location. Two visitors using Google Search in New York may get a totally different search result.
One of the results being the huge number of Google servers probabily in Millions. However, frequent change in rankings would not be a good idea to go for.