Whats confusing there are 3 voltages:

VDDC is the GPU core voltage.
MVDD is the memory voltage.
VDDCI is the I/O bus voltage (between memory and GPU core) and comes from the PCI-Express slot.

https://www.geeks3d.com/20101107/radeon-hd-6870-voltage-check-points-vddc-vddci-and-mvdd/

But yeah you can check that in GPU-Z unless it reports inaccurate settings..