VU meters are not peak meters. They are RMS or averaging meters. There are several reasons as to why you are seeing these results:1 - If you're using different VU meters, they could be calibrated differently.2 - There is no standard dBFS level that corresponds to a 0VU. Therefore, different digital devices, depending on how they're calibrated, can output different voltages for the same dBFS level.3 - In addition to the above 2 reasons, there is also a level difference between using balanced and unbalanced cables.