It doesn't exactly tell you your future PageRank at all times. Normally when a PageRank and backlink update take place, they are updated from data center to data center. This was coined the Google Dance, because of the different data found at the various Google data centers. During this update, Google updates the PageRank of your site on the various data centers. Those PageRank values at the data centers that were updated often reflect your future PageRank value on the main Google.com server.
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