Wow, I hate to have missed all the fun! Once again, Focht3 hits the nail squarely on the head. The standard Proctor is NEVER representative of the field compaction effort unless someone is using the ole horse-drawn sheepsfoot roller the test was designed to emulate. For modern day compaction (modern meaning since World War I), the modified version is the most applicable.
Personally, I'd like to see the standard variety disappear, but its inertia in the design community (you know, the folks with the blinders on?) is similar in magnitude to that of earth's...
Anyway, just remember that no soil has a characteristic "optimum" moisture content; the optimum moisture content varies with the compactive effort... as you increase the compactive effort, the moisture-density relationship shifts "higher and drier;" i.e., the maximum dry density FOR THAT EFFORT increases, and the corresponding OMC decreases.
Be forewarned: somewhere there some graduates of East Carolina University's Construction Mgmt program that actually understand this concept; I'd hate to see them show up any geotechnical engineers out there!
D. Bruce Nothdurft, MSCE, PE, PG