Recently, I found some webpages with high PR but low qualtiy and quantity of backlinks.
what's the reason
A lot of webmaster found their ranking and PR have been greatly changed. And some one speculate that Google's ways of calculating PR have been changed.
What's your opinion?
Hope you the winner of this competition

Google constantly changes subtleties of its ranking algorithm. How much weight is given to certain factors, how over-optimized sites are punished, and so on.

I believe in the end evolution will always give us the best search engine: if a search engine is spammed, we won't use. To fight this the search engine we are using will have to be successful in analyzing spam patterns (by-my-computer-supply-now-cheap.com) vs real authority patterns (cnn.com). Both are highly linked. Only one is for real. If Google one day completely fails to fight spam we'll switch to another engine, as simple as that.

The only thing people can really do is fight a niche to spam. One that is not important enough to change the algorithm for (like minor political Googlebombing), but that still works. We can see a lot of successful link farms; a webmaster combined many top-level and sub-level domains into a giant crosslinked network, with lots of pseudo-English, and search engine results, to spice it up a little. (The German Google results are even worse in these regards.)

And I bet some people are even making money out of it. Are they evil? Well, they destroy parts of our Web temporarily; in the long run, they strengthen the defense algorithms of search engines. It's a game of cat and mouse. If laws make it illegal, then so be it.

What's my opinion? Whatever you do on your own page counts less and less. Pure quantity in links counts less and less. Spammers may be more successfully punished in the future. Google may also find ways to lower the weight given to guestbook links, or link text consisting of the URL and nothing else (because this might mean the link is auto-tagged by some software, and not as valuable – spammers often choose their keyphrase in the domain name for this reason).

Google must lower the value of such links, but we need to be careful not to punish them. Why? Because then in order to hurt your competitor, all you'd need to do is create a little bot spamming guestbooks with URL www.microsoft.com or www.ibm.com or whoever you want to see punished. We can see from this that figuring out real authority on the free Web is a tough thing to do.

One future day, we might also see a kind of software helping us to sort it all out. It would take a city of trusted editors to flag URLs as "spam" and Google would have to tap the power of this database. Then again, the Web grows so quickly a city might not be enough. And as always... who watches the watchmen?