Keyword variations, the introduction of new competitors, search location, browser language, and a multitude of other factors will influence the position of your rankings. In the case of a new site, since Google does not have much information about your site yet, you should expect the results to fluctuate quite a bit until Google gathers enough information and decides on what position your site deserves.
If you want to see the average position that you blog pages are ranking for specific keywords, then create a Google Webmaster Tools account and link your website to it. From there, you'll be able to see the search data about your site.