Nothing to do with the electrical grid. It is just that the positive lead of the output of the power supply is whatever voltage you select above the negative lead on the output side of the power supply. Those are the two relative points in this situation. You can call the negative lead ground if you want. Wiring diagrams for the device you connect it to will likely call it ground. It is not the same thing as the grounding wire in a plug socket.

Note that the grounding wires in a home have nothing to do with any live circuit in your house. They are to make faults in the system safer by giving a path for electrons to flow if a conductive casing ever gets into contact with a live wire. In normal operation nothing should be flowing through it at all. With doubly insulated devices (like that power supply) the grounding wire in the plug socket is probably not connected to anything at all in the device. The plug likely does not even have a grounding wire in it.