i think its unrealistic to expect the fans to work the same at idle and during full use.
Even at the exact same voltage, whether the GPU is under load or not plays a big part in its heat output. Like I said, my overclocked 7870 is at a rock solid 1237mV at idle and in Furmark, yet in Furmark the temps pick up a fair bit. Its not just the voltage that dictates how hot the card runs, though it is a factor.
On that note, I think I'm going to see if I can lower my voltage a bit. I do find my GPU fan a bit annoying when it goes over 50% speed.