It has to be considered a bit of a science where fragments of computer code decides the ranking of your website in the SERPs. The exact elements of what constitutes your sites structure determines where your webpages end up on the search engines. So are your page results on the first page of Google or do they some how mysteriously appear on page 30.
Whether it may be indirectly related to how Google and their ever changing algorithm decides to score your particular domain as well as its pages …