my guess is that the dually rims are different. Because the dually has back tires mounted together... 1 is flipped in relation to the other. All 6 wheels are interchangeable, so the front rims are flipped in relation to the outer rear wheels. This means that a dually wheel has more backspacing than a standard wheel...in fact it prolly has more backspacing than rim width.
This probably came up because dually equiped vehicles often have an extension or adapter on the front wheel hub to push the front tires outward, to make up for all that backspacing. Hope that helps.
