Measure voltage difference between ground and DC floating power supply lines
Because the supply is floating, and you only made one measurement at once.

Floating means isolated, which means the resistance between the supply and ground is very very high. So high that the 10 Mohm input resistance of your multimeter is almost a short circuit in comparison.







simulate this circuit &ndash; Schematic created using CircuitLab

If you were to make both measurements at once, with identical meters, you would find V- at -6V and V+ at +6V with respect to ground, and 12V of course between V- and V+.