Manually set the cpu voltage to 1.2v in the bios and re check while running prime.
The mobo is auto adjusting the voltage, possibly for stability as mentioned above.
You may find @1.2v its not stable and you may need to manually bump it up until it is.
1.2v may not be your stock voltage, if you haven't chabged anything and everythings on autos 1.288v may be your default load voltage.
Set everything to stock/default speeds, load up prime95 and check the cpu voltage, that will be your cpu's default voltage.