Alexa rank is determined using the information collected from the users who have an Alexa Toolbar installed.
So, now, it won't collect the data or even take into consideration the users who don't have that Toolbar.

So, the alexa rank and statistics can be quite deceptive sometimes,
because not all here uses an alexa toolbar.

Whereas The google's PR users a fairly different algorithm for calculations a PR, so, it has higher accuracy and hence credibility also.