About Live PageRank

The Live PageRank calculator gives you the current PageRank value in the Google index, not just the snapshot that is displayed in the toolbar. Google updates its internal PageRank value continuously as the web changes and their index is updated. Only once every third month or so this value is exported to be displayed in the Google Toolbar.

What senior Google engineer Matt Cutts has to say about updates:

What is an update? Google updates its index data, including backlinks and PageRank, continually and continuously. We only export new backlinks, PageRank, or directory data every three months or so though. (We started doing that last year when too many SEOs were suffering from `B.O.', short for backlink obsession.) When new backlinks/PageRank appear, we've already factored that into our rankings quite a while ago. So new backlinks/PageRank are fun to see, but it's not an update; it's just already-factored-in data being exported visibly for the first time in a while.

The Live PageRank value may be used as an indicator of what will show when Google decides to export the PageRank values to the Google Toolbar.