According to CWLee :

Picture two wires coming from the breaker box. One of these
two wires was then connected to the input side of the dimmer
switch. Then the digital voltmeter leads were attached to
the other wire, and to the output side of the dimmer switch.
No other load involved. Voltmeter set to read AC. As the
toggle on the dimmer was moved from full ON slowly to full
OFF the voltage shown on the voltmeter dropped slowly from
around 120 volts ac to around 50 volts ac.

DVMs should not be trusted to read the output of dimmers,
_especially_ when there's no load on the dimmer.

DVMs have such high impedance, they can read phantom induced
voltages off wires that aren't connected to anything.

If you attach the bulb to the dimmer too, you will get a somewhat
more reliable reading.

Secondly, dimmers, as they get closer to the fully dimmed
out state are generating a voltage waveform that's rather
far away from a sine wave. Unless it's a very good quality
DVM, it can display grossly misleading readings.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.