You're an idiot. READ his post. He was talking about the two sides of HIS service, idiot.
You're an idiot.
Read either side of your house service at ANY time and the two voltages will be right on each other with perhaps a volt or two variance between them. Maybe when the power company puts too many houses on the same transformer, you could see a variance that is greater if the other houses are all loaded up on one side, but the likelihood id very low.
And reading them one right after the other IS "at the same time" for this purpose, ya friggin dope!
Of course taking a reading a day later could result in a differing voltage.
You have about as much common sense as a freshly laid turd.