Nate Cohn for the Upshot provides a statistics lesson in the context of election forecasts and why they differ so much.
[P]ollsters make a series of decisions when designing their survey, from determining likely voters to adjusting their respondents to match the demographics of the electorate. These decisions are hard. They usually take place behind the scenes, and they can make a huge difference.
To make the point, the Upshot gave four pollers the same raw data to interpret. Results varied.