site's ranking factor in additon to back link

fluctuations in the percentage of pages indexed is not something to worry about. (Focusing on improving clickthroughs or conversions on your top 20 URLs can often have far more impact on your site's ranking than striving to get 100% of your pages indexed.)

 

n addition, we don't index images directly (instead, we index the page that contains the image). As a result, direct image URLs in your Sitemap won't be indexed.

 

so IMO,CTR of site's page,especial the top ten or twenty important page  is taken into account for the site's ranking possition,just like for ppc possion.