Voltage is not totally locked on NV side, but they indeed limit the maximum voltage that can be set via official NVIDIA API to 1.175mV on Kepler series. And they rather agressively try to force all vendors to follow the limitation. There should be a bit more freedom in voltage control area on next gen NV hardware. But all those limitations apply to official NVIDIA voltage control API and AMD have NO voltage control API at all (and even no reliable I2C access API, so vendors need to create low-level I2C driver for each new AMD chip). So in the end it is much easier for software developers to provide working and reliable voltage control for NV hardware, even if NV try to limit it.