Basically, he's bending over backwards to make sure he isn't wrong again about Trump. Remember, Silver said there was no chance Trump would get the nomination.
"To see how it works in action, take the Marist College poll conducted Oct. 25-26. Silver rates Marist as an “A” pollster, and they found Clinton with a 1-point lead. Silver then “adjusted” it to make it a 3-point Trump lead."
http://www.huffingtonpost.com/entry/nate-silver-election-forecast_us_581e1c33e4b0d9ce6fbc6f7f