It only looks like the last poll governs because there have still been so few polls.
His algorithm is explained here: "The most recent poll is always used, and, if any other polls were taken within a week of it, they are all averaged together."
When polls are coming thick and fast, more will be used. For example, check out this page from October 21, 2012, and mouse over each state. There wasn't much interest in polling Hawaii so the site was still using a poll from September 28. Minnesota had two polls counted, so the projection was shown to be based on "Survey USA+PPP". For Florida, the main map says only "Six polls", there being too many to fit in the little box, but you can left-click on Florida and see all the poll results throughout the campaign.
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