I know this is not applicable to all BMW voltmeters, but I got one off of eBay, and checked it's accuracy with a DC power supply .
I had a digital voltmeter, connected to the terminals on the BMW voltmeter, I set the power supply output to the corresponding graduations on the BMW voltmeter, and the BMW indicator, showed within a needles width the correct voltage .
I selected various points on the bikes electrical system, to sample the voltage, and this is where I believe the voltmeter inaccuracies are coming from .
Where you wire in the voltmeter, is a big determining factor, on it's accuracy of indication .
The closer to the battery, the more accurate the indication .
Thanks for the pictures, up to now, I could not figure out how the meters were mounted, just by looking at the 'exploded' diagrams in the parts catalogs .