Measuring output voltage (Single ended probe vs differential ended probe)
One potential problem with a single-ended 'scope probe is the loop area formed by the probe plus six-inch ground clip. Any high-frequency alternating magnetic field entering this loop area generates a voltage fed back to the 'scope:

When you're probing a switch-mode power source, this loop area may be very near an inductor whose chopping frequency is not well-contained...noise!

Some good probes include a small springy fixture that attaches to the probe's tip that reduces the loop area greatly...the ground connection is annoyingly short and requires some dexterity to get probe tip and short ground both connected:

There are other ground-loop scenarios too, involving your scope (which is earth-grounded for safety) and the device that you're probing which is also earth-grounded...that's a huge loop that may add much 50/60 Hz noise to your measurement.