I certainly don't fit the title of Google PR guru, but I would GUESS that it has to do with sites linking up with the www in there. I have a few mainstream sites where I see the same thing. (pr5 to pr2).
One would think that Google's algo would ignore the use of the www or not, but it would seem that it doesn't. I bet it has to do with the use of the third level importance.
I'll be curious what others have to say about this.
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