This is a result of different measurement systems that are used to calculate the capacity/volume. The ENERGY STAR Web site lists volumes based on the U.S. Department of Energy (DOE) test procedures, which match the volumes printed on the FTC (Federal Trade Commission) yellow EnergyGuide labels. Historically, some manufacturers and retailers have show volumes based on the IEC (International Electrotechnical Commission) test procedures, which calculate volume differently for models that do not have a central agitator. You may see front loaders or newly redesigned top loaders without a central agitator will have a slightly larger capacity in the retail product literature.
Beginning in May 2011, most manufacturers will only advertise clothes washer volumes that are based on the U.S. DOE test procedure, so the volume you see on the FTC EnergyGuide Label and the ENERGY STAR website is likely to be the same as the volume advertised by manufacturers. More information about ENERGY STAR qualified clothes washers.