While your project got the right result, I don't think it's mathematically justifiable to take an arithmetic mean of the published survey means without accounti

While your project got the right result, I don't think it's mathematically justifiable to take an arithmetic mean of the published survey means without accounting for the standard deviation of each sample distribution. Doing that would make your model more mathematically complicated, but it's no doubt one of the things that Nate Silver is doing. But besides that, Silver isn't just trying to call the election the Monday before the election, he's trying to build a model that predicts the outcome as early as possible. A lot of the stuff he's been throwing into it, like the 'economic factors' or 'convention bounce' adjustments are about keeping the model from going off into the weeds due to predictable fluctuations in public sentiment caused by ephemeral events. If you compare the history on the right column of the site to the "now cast" tab, which excludes those adjustments, you'll notice that the former contains fewer and more modest fluctuations than the "now cast".