Further, by this point in 2010, the polling was much more consistent than we see now. The σ at this point was 1.99% compared to the current 4.50%. This shows that the generic ballot polls were much closer together in result than they are this year. Though, this year the DEM values (as a stand alone number set) are more consistent than they were in 2010.
The generic ballot polls have been a good indicator in past cycles of how the election will turn out. While there are always individual races that don't follow a national trend; this overall number gives a good indication of who is going to show up on election day.