Why is the measured mains voltage does not match the mains voltage readings according to the HMI on ATV61 or ATV71?

Issue:
Why is the measured mains voltage does not match the mains voltage readings according to the HMI on ATV61 or ATV71?

Product line:  
Altivar ATV61 and ATV71

Environment: 
All models, all serial numbers

Cause:
Flat topping on the input AC waveform. 

Resolution:  
The HMI's Mains Voltage reading is a calculation of the Mains Voltage, by reading the DC bus. It is calculated by taking the DC bus value and dividing it by the square root of 2.