agreed, this is usually happen i remember i've downloaded leaked image of win8_rtm.iso final/genuine after 24 hours its signed-off in M$ labs, same happen with win7_rtm final... and all thx goes to great leaker "ZUKO" who make's us HAPPY
Don't get too excited, the true final won't have a build number like 9484 etc, it will be 9600. If it does end up being some obscure number, Microsoft have changed the way they do things.
And that is not out of the realm of possibilities with the latest MS changes from the norm. Hard to tell for sure.
The sign-off candidate will carry a minor build No.(delta, QFE code), starting from 16384. You don't see it, it is not a RTM sign-off candidate.