cloudtap - 3:35 pm on Jan 6, 2011 (gmt 0)
The primary reason is that Google does not want to give users their exact methodologies due to more manipulation and having the PR be too accurate would also be too burdensome on the server farms of Google. Google basically changes the PageRank of pages constantly, probably atleast once a week. Their visible PageRank doesn't change until about every quarter because changing that much data based upon the algos not only gives away their true data but also is a HUGE stress to load balancing. Imagine giving trillions of pages constant visible updates to their PRs daily to weekly, EVEN monthly -- it's too difficult.
That doesn't also include the effects of personalization algorithms changing the PR constantly as well as the social graph effects. Lastly, the PR of a page could vary from server to server based upon your location.
The closest server farm to you is serving you your SERPs, the SERPs change almost constantly past page 2 across different areas dependent on where you search.