Apparently my P20 is working well, even if I noticed this issue.
Please, would you be so kind to verify with a voltmeter what reads in your P20 between neutral and earth?
This should be very helpful for me.
Thank you very much
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I I see where the problem lies! 
I had connected my P20 with the phase reversed to the wall socket as it seemed to sound better. When wired out of phase, the P20 creates a voltage difference between neutral and ground of about 18 volts.
If I connect the P20 to the wall socket with the correct phase, the difference between neutral and ground becomes the same as that of the wall socket.
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Asking the obvious, which orientation sounds better and in what way?
Based off this image the P20 is working very well.
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I am delighted you found the reason, but can anyone explain why the different phase connection would make a difference when connecting neutral and ground?
So the line neutral is carried to the load outlets? Yes, it can be done that way but why not create the load neutral from the DC supply and bond that to ground (which would be required in this scenario)? Then you would have complete isolation frorn the AC mains.
Saw this thread while I was looking for answers in regards to the same issue. I have the exact same issue with a powerplant and i am located in Turkey but the exact same thing happens with every kind of mains block i tested including a ups.
If the phase and neutral are reversed at the wall connection the voltage between neutral and earth gets over 15v but if I check the orientation before connecting to the wall it stays same as the wall which is 0.7. not sure what causes this.