On Master L/R output meters, is 0 always +4dBu?
Elsewhere is the signal path, DSP for example, 0 is 0dBu (.775 Volts).
Why are these Zeros indicating different voltages?
Actually there are more differences than that. 0dBu, 0dBV and a number of DSP's have their outputs "calibrated" so that they are reading how much level the output is before you reach the threshold of the limiter.
So it doesn't give any real indication of actual output level-unless you know the threshold of the limiter-and even then-it might be before the output gain-which could change the level even more.
You really need to know what the particular gear you are working with is calibrated to.
It is not that one is wrong-but there are different ways of looking at the same thing.
Such as with weightings on an SPL meter. Your ears say it is a certain loudness-but the meter can read very different depending on whether is on A or C weighting-fast or slow response etc.
None of those change how loud it is, but do give you a different way of analizing the actual incoming signal.