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Google exports the internal PageRank values from its index to be displayed in the Google Toolbar about every three months. There is a lag of about two or three weeks from when the snapshot was taken till it's visible on the toolbar. The evidence for this is quite clear.
The PageRank of a web page undergo constant reevaluation, even if the content itself is not being updated. Inbound links come and go, and the PageRank of linking pages fluctuate, and this causes a flux in the live PageRank value. A mature and well established web page will not experience this flux on the same level as a more recent web page, or if the page has been recently discovered by the Internet community at large or undergone a search engine marketing campaign, which would cause a boost in the number of inbound links.
Sites that are still growing and publishing new content regularly will surely see a difference in Toolbar PageRank and Live PageRank as the new web pages were not even around at the time of the last Toolbar PageRank export, so the new pages will be PR0 if you go by the toolbar. The Live PageRank of the new page will be instantly updated as soon as Google stumbles upon anything that would cause a PageRank calculation parameter change, like internal links on the site on which the web page exists, or a newly found inbound link from another site, or updated PageRank for the web page linking in.
As mentioned above, there is a lag of about two to three weeks from when the snapshot of the internal PageRank is taken till it is made visible on the toolbar. This will cause the most effect on very new content, as the link building during the 2-3 weeks will have a huge impact on the PageRank value. It will also make a visible difference if PageRank is just on the border of say PR4 and PR5, and the flux causes it to go up and down between the two values.