The rated voltage is an RMS (root mean square–they square the value to make sure it is always positive, then average it, then take the square root of the average to make up for having squared it in the first place) measurement. Most multimeters are designed to give correct RMS readings when applied to sine waves, but not when they are applied to other waveforms. They will read from 2% to 20% low in voltage. Look for a voltmeter that brags about “True RMS” readings, and that will read correctly no matter what the wave shape is.