The measurement the meter displays is not in Volume Units. Therefore, it is not a Volume Unit meter. It is something else.
The VU meter -
https://en.wikipedia.org/wiki/VU_meter
VU is a name commonly misused for other meter types.
The "meter" displayed might be called a power meter - showing watts, and decibels relative to the maximum output power (which will be a vague guess, since I think I safely can assume the load is indeterminate and unspecified and in the case of a speaker, somewhat variable).
Better yet might be to redesignate it an Output Voltage Meter with appropriate scale markings. The Output Voltage can be accurately measured, independent of the load variation, and a decibel scale could be applied to that reading.
The misnamed "VU" meters on my old
Pioneer SX780 show watts and dB. Interestingly, they show lots of watts even when no speakers (load) are attached and no power (current is zero) is being generated. This is surely at the root of my general dissatisfaction of flashy meters.
A somewhat scholarly discussion of VU -
http://www.aes.org/aeshc/pdf/mcknight_qa-on-the-svi-6.pdf