Its overvolting because of the modified vBIOS imo. Drivers don't cause gpu's to automatically over volt. Either you do that manually or takes the value of the current installed bios profile.
I hope you know that drivers control the voltages through interaction with the vBIOS. With Kepler, drivers are significantly more in control of the voltages after Nvidia locked out the user from extensively overvolting his/her GPUs. Only the first batch of MSI lightning edition GPUs had true voltage tuning.