Why do my appliances show two sets of power consumption ratings and which one applies to my house?
Assuming you have the split-phase power supply that is normal for residential settings in the United States, you should use the line item for 120/240V. 

The 120/208V line is for 3-phase supplies, which you typically find in commercial or industrial settings (each leg of a 3-phase supply is 120V relative to the neutral, and is at a 120&deg; phase difference to each of the other legs, so the voltage between them is 120V * sqrt(3) = 208V).