It is the same plane 99% of the time. But there is always the chance of a mechanical or some other event happening where CO can swap in a replacement aircraft if they had to by flying one up from IAH, I suppose.

If you book it as ANC-EWR you'll likely run in to trouble with EUAs and seat assignments. And you'll be credited with fewer miles. Book as ANC-SEA-EWR for better results on all three fronts, in exchange for a couple dollars in additional taxes.