It's difficult to tell the real PageRank value, the bar is just a guide.
According to Google's research paper "The Anatomy of a Large-Scale Hypertextual Web Search Engine", the figure is calculated based on the page that links to you. However, there may be other new calculations added since then, based on context, age of the link, etc.
http://infolab.stanford.edu/~backrub/google.html
Also check out:
http://www.v7n.com/forums/seo-forum/...-pagerank.html