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I suppose because a 301 is an instruction that you've permenantly moved a resource and you are effectively asking for those ranking signals associated to be migrated across to the new location. If it is to be close to PageRank in its approach you would expect a migration of that link equity. In the case I'm looking at, one year on, the page that has been redirected almost has the same PA as the new page.

PageA.html has a lot of links and has a Page Authority score. It is 301 redirected to PageB.html and eventually in OSE this also accumulates a Page Authority score. Is there a reason why PageA.html many months/years after continues to have a Page Authority when it is expected that its Authority signals have migrated (with any dampening effect etc) across to PageB.html.