Two items that may be important:
First, are you measuring the equipment in isolation? That is: nothing else is connected to the component being tested?
The equipment should be in isolation, even all the interconnectors off!, or voltage from other equipment will skew your results.
Second, are all your components plugged into the same wall outlet (both plugs or four way unit of one service recepticale?) if not, then you also may have a problem with the different outlets being wired differently (you can check this by using an ordinary light bulb in a socket and two probes: insert the probe into one channel of the wall's outlet and the other to ground, then test the other wall outlet A/C opening and ground,... one should light the bulb... all you outlets should have the same side light the bulb when the other leg is to ground.
Secondly, check if the 'hot' side of multiple outlets have a 220 voltage differential. If the two different outlets are using different "legs" of the standard 220 three wire house voltage, you could get some odd stray volatage readings (I am not explaining this properly as if I were an electrician, but I hope it makes sense)
And I would NOT change the wiring to reflect you units preference, I would open the unit up and swap the wires at the units A/C connection to the A/C wire, or to the chassis IEC connector.