Google PR and Alexa rank is different mainly because alexa focuses on the traffic on a specific website, but for google PR there's a lot of factors, I would name a few so that would be if the site is properly SEO optimized (that includes submitting data to webmaster tools,proper permalinks/redirection) and not using any black hat methods (cause google not to trust your site), page load, quality contents and not just duplicating contents of other sites, etc.