This is what the USDA website has to say about how the new map was created:
The zones in this edition were calculated based on 1976-2005 temperature data. Each zone represents the average annual extreme minimum temperature for an area, reflecting the temperatures recorded for each of the years 1976-2005. This does not represent the coldest it has ever been or ever will be in an area, but it reflects the average lowest winter temperature for a given geographic area for this time period. This average value became the standard for zones in the 1960s. The previous edition of the USDA Plant Hardiness Zone Map, which was revised and published in 1990, was drawn from weather data from 1974 to 1986.
In the Eastern US (my part of the country), the weather data used in constructing the map are from weather stations of the National Weather Service.
The new map places most of Atlanta in zone 8a. My town, Kennesaw, which is not labeled on the map but is a little northwest of Marietta (identified in the Georgia map - "Click" on a state to see it in more detail), is placed in zone 7b, which has lowest temperatures of 5 to 10 degrees F.
However, the 2006 Arbor Day Foundation Hardiness Zone Map places Kennesaw in zone 8. The Arbor Day map is based on low temperatures from the 15 most recent years' worth of data that were available at the time, using data from National Climatic Data Center cooperative stations.
Although both maps show that climate zones are shifting - with winter lows getting less low nearly everywhere on the continental US - there still seem to be areas of disagreement. Since I'm not a climate scientist or a weatherman, I don't know why the two data sets produced such different results, but the disparity means that gardeners who are looking for plants that will do well may need to be especially aware of the micro-climate of their own yards in making the final determination of what to plant.
Is my yard in zone 7b or zone 8? For most annual vegetables, the difference isn't big enough to be too worried about, but I have been using zone 8 as my benchmark when making choices about plants that require a certain minimum of "chilling hours" to set fruit.