Some of the USAir registration numbers for 737s also end are N***US. However being that the US is a popular two letter code, Northwest uses it quite frequently, USAir wasn't able to register all of it's aircraft in the same three digit number field with the US ending. Take a look at the new US Airways Airbus fleet. They are N***UW. They gave that reasoning above to keep all A319 & A320 aircraft within the same three digit number field.
Also you will see some other airlines, such as Frontier, Vanguard, old WestPac, that end in AU. They were old USAir aircraft that were either sold or returned to the leasing company by USAir and never changed by the new operating airline. The same also applies to the old USAir F28-1000/4000 fleet, ie: Horizon Airlines.