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Here is how FiveThirtyEight reports their forecast probabilities.
FiveThirtyEight Clinton Trump
--------------- ------- -----
Polls Only 51.0 49.0
Now Cast 56.8 43.2
What explains the difference between the Polls-Only forecast and the Now-Cast forecast?
The numbers you're comparing are not "poll results", they're odds of winning based on the polling.
The details of their models are spelled out here. They summarize them as:
Polls-plus: Combines polls with an economic index. Since the economic index implies that this election should be a tossup, it assumes the race will tighten somewhat.
Polls-only: A simpler, what-you-see-is-what-you-get version of the model. It assumes current polls reflect the best forecast for November, although with a lot of uncertainty.
Now-cast: A projection of what would happen in a hypothetical election held today. Much more aggressive than the other models.
Then, they even have a section called "Differences between polls-only and now-cast":
- The now-cast is basically the polls-only model, except that we lie to our computer and tell it the election is today.
- As a result, the now-cast is very aggressive. It’s much more confident than polls-plus or polls-only; it weights recent polls more heavily and is more aggressive in calculating a trend line.
- There could be some big differences around the conventions. The polls-only and polls-plus models discount polls taken just after the conventions, whereas the now-cast will work to quickly capture the convention bounce.
