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Here's the issue. If you're thinking about restricting links out that you'd normally place, because you're afraid of leaking PR, don't. Such behaviour negatively effects the natural karma of your page, and despite my use of the work 'karma,' which could be taken lightly, artificially limiting links out is bad. Why? Because if your site would naturally link out a lot, and you don't do so, then your site, then your site becomes a 'camel' when compared with others if it's kind, and may end up seeming artifical to G. Also, by limting what your site 'wants to be', you limit G's ability to understand what your site really IS.
Conversely, if you just throw in a bunch of outbound links on pages because you're thinking G or Brett wants you too, that is potentially problmatic too (unless you're really clever). Although, a few good links out, if done with good intent (i.e., useful to the surfer) never hurts. ;-)
Look at most of the pages on this site btw.
The bottom line is that if your site is a directory, then it better have a bunch of links out, or it will look like a faux directory. If it's a content site, try blending the links out into the copy, or neatly juxtaposed with the copy. Make it useful for the reader.
For all the winging and moaning about lost sites in here etc, I've rarely seen a very good content site, or a very good directory site that doesn't do at least OK in G, over time.
These are not highly advanced points of advice, but from what I've seen, most would do really well to truly focus on the basics before moving up the SEO ladder. As is true in nearly all aspects of business, 80% of those playing with SEO fall into the middle or bottom of the bell curve, usually for simple lack of mastering the basics (not as easy as it seems), or lack of attention to deal. FWIW.
There will probably come a time when there are actually too many good sites out there in many categories, such that not all really great sites see the light of day in G's SERP's. But we're not close to being there, yet.
All that said, the answer to your question is: A page can have PR 7 because it receives enough inbound links to become a PR 7. It's as simple as that.