Seems to be a flyback converter with little or no EMI filtering components?
Consider a typical EMI equivalent circuit like:
![](https://www.seventransistorlabs.com/Images/EMI_Filter.png)
There is some voltage across the isolation transformer, and thus through its isolation capacitance. This is V1 and C3. There is usually a shunt capacitor (between pri/sec GNDs), C7, which must be Y1 rated. The secondary to ground/earth path, and primary to earth or secondary ground path, varies between devices (hence C6 is shown shorted: take your pick, sometimes it's direct connected, sometimes it's isolated and RF-grounded through a capacitor). It's reasonable to assume there is no (or insufficient) secondary-to-chassis path here, given the amplitude shown; or that the primary-side choke is woefully insufficient.
The effect further extends to the scope probe. Consider the ~0.2uH series inductance of the ground clip, through which CM current flows (up to the scope body, and perhaps onward to earth or mains, back through the wiring, to the PSU primary side, closing the CM loop). Whereas CM current is NOT flowing through the probe tip (it's high impedance at these frequencies), thus the voltage drop on the ground clip is subtracted from the probe differential measurement, and you cannot measure the differential voltage alone, not with generic probe technique anyway.
Tim