This means how updated your page content is and currently what is the trust for that page....these two things Google consider in PR ...that's why its not constant....
PR is all about links (Inbound Links) and Google keeps on updating the REAL PageRank all the time, but they only export one spanshot of their internal PR values once in a while.. which might be out dated
PR is base on back links and other factors. I think now google doing some changers to update real time PR. Internal PR update going on around 2 months now.
your PR is never the same, your PR is calculated by how many links you have and how much they are worth, it changes everytime google updates.
Page Rank can remain fairly stable if you are a static site with thousands of links, built carefully over a long period of time. I got PR 4 on the last update (Dec 31, 2008) and expect to reach PR 5 in another two years (when I reach 10,000 links).