It doesnt PREDICT the pagerank, it is telling you that currently your pages have a low (1-3, what I believe) pagerank, as you will go through getting more back links, it will increase..
It does what it says. It gives you a clue about if your pages are High, Medium, Low or unranked. So its only really a clue and I dont think it gets updated very often.
This tool is really awesome. I use it a lot, especially for new sites. One of my sites was blacklisted (it was an expired domain I picked up) so it has had a pagerank of 0 forever (over 1 year). Now the bars display medium page rank for almost all of my pages and low PR for the rest. All this with only minimal unassigned. Thanks for reminding me about this useful tool!
It updates at least once in a month.. Well that depends, may be your site is new, or is being panelized by google, did you successfully verify the site?