By default, the zero of energy in the device DOS is set to the average of the Fermi Levels of the left and right electrodes, and the device DOS is just for the center region. http://quantumwise.com/documents/manuals/latest/ReferenceManual/XHTML/ref.devicedensityofstates.html
When the gate voltage or electrode bias voltages are applied, the device DOS should change with the applied voltage due to the charge injection of gate or the shifting of the Fermi levels in two electrodes. It is sure that the device DOS can be used to explain the I-V curve or the transmission spectrum. But it is not alway obvious.