Bench DC power supply output
The ground terminal is not half way between negative and positive pins it is ground i.e. connected through to your building's earth wire. It should be isolated from both outputs (black and red).

However, an isolated power supply is still capacitively connected ( a few tens of pF) to the incoming AC live and neutral - the net effect is these caps form a potential divider reducing (when unloaded) the 240v to exactly half i.e. 120V - this won't hurt you because the capacitance is so small however, your meter's input impedance is maybe 10Mohm and it will register the voltage.

Try putting a 100k resistor between red terminal and ground and re-measuring - I bet it falls to just a few volts.