Alexa is a traffic based site....maybe the site you are talking about have no PR yet but some real high traffic(due to some reason) that is why there Alexa ranking is way up.
As khushal said, pagerank and traffic aren't always directly related. They could have high traffic through a couple of well placed ads, or, they could be faking their alexa ranking with traffic exchanges or something similar.
PR doesn't have any relationship to Alexa .. I wrote about on my article about how to increase your Alexa Rank.
Higher pagerank doesn't mean you can rank high at alexa.There are different terms between Alexa and Pagerank.
Basically, both PR and Alexa are the wrong metrics for website popularity. Both have serious flaws and can easily be manipulated so don't lose any sleep over any of the two
well... as you know alexa provide traffic information and popularity charts using data from it's members toolbar (Alexa Toolbar), which is most used in US. That's why there are such differences.