Does your DMM give you accurate readings? Unless you're using a True RMS meter, probably not. Switching power supplies and other sources of waveform distortion are everywhere. An average-responding meter gives you an inaccurate reading where distorted waveforms are present. In fact, such meters can't even read non-sinusoidal waveforms.

Because of this deficiency, it may seem average-reading meters are useless for commercial and industrial maintenance. However, there's a good use for these meters. Remember, they don't read the non-sinusoidal waveforms, but True RMS meters do. Take a voltage reading on a circuit, using one of each type of meter. The larger the difference in readings, the more significant harmonic distortion is. For all other maintenance applications, use only DMMs that say "True RMS" or "T-RMS" on the meter case.