Of course 538 would adjust this down to Clinton +3 even though almost every other ABC poll in the past has been adjusted up. What a joke.
Was just about to post this. Interesting decision.
It's not a "decision". These adjustments are made based on a statistical model (I'd guess something like an OLS regression, though it might be something more complicated) that takes into account factors such as trend and house effect. I don't know how they got to this adjustment (I'm surprised by it myself), but this is not a judgment call by Silver or anyone.