Difference between 0-18V and -9V - +9V
In this case  BAT1 has at the plus side 18 V, and at the BAT2 minus side 0 V, because GND is connected there.







simulate this circuit &ndash; Schematic created using CircuitLab

Below, the GND is in between the batteries, so on either side the difference is 9V, resulting in +9V at the + side of BAT1 and -9V at the - side of BAT2.







simulate this circuit