Measure Lab DC Power Supply with oscilloscope
Many lab power supplies have "floating" outputs - neither output terminal is connected to chassis or AC power ground.  I would expect such supplies to have three terminals on the front panel: +, -, and Ground, with the Ground terminal connected to the AC power ground, and the actual power supply output connected between the + and - terminals.

If you connected your scope ground clip to the supply ground terminal, and the scope probe tip to the supply positive terminal, I would expect meaningless readings on the scope, as there is no defined relationship (no connection) between the supply + terminal and Ground.