The stated battery capacity is a rough estimate. There is no reason to be concerned, my PowerBook's battery says 4600 when it should be something like 4400. Calibrating your battery will only give a more accurate reading.
I understand that all new batteries are tested. If they met or are ABOVE the standard, the MBP being 5600, they are used. If below they fail. You just happened to get a battery that reads above the standard.