Comparing the polls

On August 4, Public Policy Polling had Bob McDonnell leading Creigh Deeds 51-37.
September 1, they have McDonnell's lead down to 49-42. What's the differences in the poll?
The August survey had 35 percent Republicans to 32 percent Democrat.
The September survey had 38 percent Democrats to 31 percent Republican.
Among area codes, the September survey took 24 percent from 757 (Hampton Roads) instead of 17 percent in August.
My area code, 540, went from 25 percent of the poll to 20 percent in September.
The August breakdown by philosophy was 45 percent conservative, 41 percent moderate and 14 percent liberal.
September has 36 percent conservative, 46 percent moderate and 19 percent liberal.
Seven points is a bit of a drop, but use the August polling sample and it's probably a 50-41 McDonnell lead.