Unfortunately this behaviour can be seen in a number of 7970 DCII cards. Multiple people across different forums reported similar voltage-locked cases. For other people everything works fine however, apparently there are two slightly different versions of DCII cards available: one with free voltage control that was given to all testers and some customers, and one with locked voltage (meaning that the maximum allowed voltage equals the default voltage, like in your case. 1100 mV defaut voltage is fine btw and well within safe values. However, you can't increase your gpu voltage for overclocking purposes. Usually the slider should be somewhere in the middle with the absolute maximum being something around 1300 mV.)
My DCII is locked at 970 to 1060 mV GPU voltage, and I've tried every version of GPU Tweak available (funny thing: the version of GPU Tweak that came with the card could only adjust core frequency, nothing else), as well as Sapphire Trixx and MSI Afterburner, all in combination with different versions of Catalyst. But nothing works.
I contacted Asus support which made me fill out a support form with lots of serial numbers from my graphics card, mainboard and so on. They even suggested a couple of things like leaving power target at default value and so on, but nothing worked and after a while they no longer seemed to care. It's a real shame because it's a mighty fine card and mine achieves 1100 MHz even at 1060 mV and due to full-cover watercooling it barely reaches 45 °C during summer, but thanks to this very restrictive gpu voltage limitation I can't go beyond 1100 MHz and see what the card can really do 😞