How does the mean-reversion adjustment work?
There has been an extremely robust tendency in Presidential elections for national polling numbers to revert to the mean as the election approaches – that is, for the trailing candidate to gain ground. The further we are out from the election, the more tightening in the polls we can expect. For example, a 20-point national lead held 200 days before the election projects, on average, to only about an 8-pont victory on Election Day, whereas a 5-point lead held 60 days before the election projects to about a 4-point victory. This adjustment is described in much more detail here.
