In most cases, the numbers are not an "average" but rather regression based trendlines. The specific methodology depends on the number of polls available.

If we have at least 8 public polls, we fit a trend line to the dots represented by each poll using a "Loess" iterative locally weighted least squares regression.
If we have between 4 and 7 polls, we fit a linear regression trend line (a straight line) to best fit the points.
If we have 3 polls or fewer, we calculate a simple average of the available surveys..

There are several principal ways that the FiveThityEight methodology differs from other poll compilations:

Firstly, we assign each poll a weighting based on that pollster's historical track record, the poll's sample size, and the recentness of the poll. More reliable polls are weighted more heavily in our averages.

Secondly, we include a regression estimate based on the demographics in each state among our 'polls', which helps to account for outlier polls and to keep the polling in its proper context.

Thirdly, we use an inferential process to compute a rolling trendline that allows us to adjust results in states that have not been polled recently and make them ‘current’.

Fourthly, we simulate the election 10,000 times for each site update in order to provide a probabilistic assessment of electoral outcomes based on a historical analysis of polling data since 1952. The simulation further accounts for the fact that similar states are likely to move together, e.g. future polling movement in states like Michigan and Ohio, or North and South Carolina, is likely to be in the same direction.

Methodology
We take current state-by-state polls and turn the results into probabilities. Each simulation picks a winner in each individual state, based on the probabilities for that state. For example, if McCain has a 55% chance of winning Nevada, he will, in the long run, win Nevada in 55% of the simulations conducted. States that are not polling close (e.g., Utah or Rhode Island) will always yield the same result. As a result, a closer election, with more swing states, will yield a wider range of simulation results than an election with fewer states that are up for grabs. The simulator does not consider the possibility of split electoral votes in Maine or Neb

Democrats are outvoting the GOP by a margin of 2.5-to-1 in North Carolina, where early voting has been under way for a week. That's roughly double the margin from 2004.

More than 210,000 blacks who are registered as Democrats have cast early ballots in the Tar Heel State — compared with roughly 174,000 registered Republicans overall. Four years ago, the number of GOP early and absentee voters was more than double that of black Democrats.

"It's a sign about how energized African-Americans are about this election," says David Bositis, who tracks black voting trends at the Washington-based Joint Center for Political and Economic Studies.

In Louisiana, more than 31 percent of the early voters are black, and Democrats are topping Republicans nearly 2-to-1. In the crucial battleground state of Florida, nearly 55 percent of early voters are registered Democrats — well above their 41 percent share of the electorate in the Sunshine State.