He had inside access to the campaign and had access to a lot more polls.
Probably.
My guess is that Silver was played like a drum by the leftist shills that are the British media. Garbage in, garbage out. He’s a credulous liberal when it comes to other liberals.
There’s a reason games are actually played and elections are actually held. A model that is accurate has to be as complicated as the reality it is describing, and no ‘model’ is.