It's all relative!
It really doesn't matter how the volume control is labeled. It performs the same function regardless. However, if it really bothers you, there is an explanation.
The McI had a relative scale starting at 0, which was total lack of volume or 100% attenuation on the source signal. Then the scale went up from there. The volume at 8 is louder than the volume relative to the 7 position.
The NAD has a volume control that is inversely relative to the 0 position. The 0 position may be a unity gain position, which simply means that the pre-amp section of the integrated amp is providing no gain at that point and the amplifier is receiving 100% of the source signal. Anything less than 0 is attenuation of the input signal.
Theoretically, the 0 position is the same volume that would be acheived if the source signal was connected directly to the amplifier without a volume contoller in the circuit.
I'm no guru, but that's the way I see it......