your assuming that the population distribution is even across the entire continental US. Clusters form which would results in some groups dealing with larger amounts of zombies than others in rural areas. As such, a large percentage of the surviving groups would die/turn in the first few months as they would not be able to cope with the density of zombies in or near metropolitan areas. The rural groups would survive but they would not be able to attain the killing numbers as described because theres just not enough people in those areas to begin with. Thus the actual killing numbers would vary significantly and the zombies would continue to roam because of this.