[keepsharing]

well, firstly a PageRank is not for a whole site, It is measured on per page basis. Its just a graph representation of the amount of indexed incoming links that Google recognizes for that particular page.

where as rankings are more complex which represents the relevancy of that content for that particular keyword.

higher page rank doesn't always mean good rankings, you can check like this:

try to make page linked with only incoming links from images, and don't keep any alt tags in the images also. once you have good number of links, that page will have some rank but will never come in SERP got that :)