Different tasks utilize the GPU to different extents. More recent applications may be slightly more powerful and thus more demanding in terms of power requirements. Some cards can run with reduced voltages and others either have to have their Voltage tweaked up slightly, or their clocks reduced (usually/mostly memory, as this has the least impact on run time).

During the lifetime of a GPU (say 3 to 5years) I would expect some GPU's to see some degradation such that the GPU would need to be tweaked slightly to be able to handle the same tasks. Either a slight increase in Voltage or a slight reduction in frequencies. As always reduce the GDDR Memory freq. first as this impacts the least on the run time. I would then look at the GPU core, if need be, and lastly the shaders.

As far as I am aware GPU's are tweaked individually in the factory, so you are likely to see different voltages in use on otherwise identical cards. So use other peoples frequencies and voltages as a guide only.