80PLUS.org lists a set of efficiency requirements for correct supplies for both 220V and 115V main power voltages. However, for the 220V ratings, all manufacturers are held to a slightly higher standard of efficiency, and the highest rating is not possible for typical US household (115V) current.
Is there something inherently more efficient about 220V power which makes it inherently more efficient and therefore deserving of more stringent standards? (Of course, the impedance load induced by wiring might be higher for 115V voltages; though I don't think that ends up being too large at typical power supply amperage and line impedance. I could be wrong there though...)