Google is again updating the published page ranks. PageRank changes daily (inside of Google). About four times a year Google updates the pagerank that is published in the toolbar. So for example, a new site for us that offers free CSS web site templates, shows a pagerank of 4. The fact that it showed no page rank last week and now shows a pagerank of 4 doesn't mean it increased in the last week. It has been obvious for some the site has been getting increasing traffic and likely had a reasonable pagerank. Now Google has just published it.
You can use another new site to the curiouscat.com family to check the pagerank for your sites: Multi PageRank Checker (pagerank 1 by the way). If past performance repeats with this update of pageranks the ranks can often move around for several days before settling down to the new rank that will be displayed for several months. Some of our sites:
Curious Cat Investing and Economics Blog - 4
Curious Cat Engineering and Science Blog - 5
curiouscat.com - 3
externs.com - internship directory - 4
Statistics for Experimenters - 3
Pagerank uses a logarithmic scale - so a pagerank of 5 is 10 times greater than one of 4 and 100 times greater than one of 3. Also say you have a 5.1 that would be 10 time greater than 5.0 - but Google only publishes 1 digit. So the real difference between pages shown as 4 and 5 theoretically could be anywhere from say some tiny percentage to nearly 100 times (if it were the lowest possible 4 and the highest possible 5). It seems very likely Google uses a logarithmic scale or something very similar but I don't think they have confirmed this publicly. Some people think perhaps there is a difference of say 6 times for each integer so a pagerank 5 is 36 (6*6) more than and pagerank 3. That is certainly possible.
Related: Google's Search Results (should factors other than user value be used?) - Google's Displayed PageRank - Web Page Authority - Are Paid Links 100% Untrustworthy? -