The reason for the difference in PR

Personally I find it very mysterious to, I want to know about the policies of search engines of Google. I met a number of issues were working on improving the PR of my site. If it is determined that with respect to the high volume of traffic and low competition keywords was good. Well, building backlinks, Yahoo helped to get a PR5 and I ding. But I find that Google has a PR of not surprised. Reasons for such differences or anyone tell me anything? Google recently changed the policy?

There are two updates from Google which is adopted of Caffeine and Panda , which you may know. Along with this two, there are large no. of minor updates which is for displaying results in Google. Hence you have good difference, you need to dig a little deeper and need to recognize the reason behind it.

Google update its pr after every three months.
You have to do proper seo to maintain your pr.