<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">RMS means Root Mean Square, which doesn't really mean anything to us. </div></BLOCKQUOTE>
Oh but it should! Take it backwards: First square the measure so you have all positive values, find the mean (average), then take the square root to get back to your original reference. That process is needed when you are averaging the impact of negative and positive voltage together like in AC power so you can compare it to DC power.
The issue noted here, about numbers changing as the meter monitors the line, has more to do with the averaging time than with the method of measurement. For an AC line monitor you generally want to average a second or two (or longer) and to show a running average. For a test instrument, you'd probably want a shorter interval. For a UPS you'd want measures that got down to one, or even a half, an AC cycle (that's why switchover numbers like 17ms are often found in inverter specs).
Note that a true RMS meter will show that an MSW inverter is providing a proper voltage. Meters that don't do the RMS thing will often show a lower voltage but may be off high, too. Wierd voltage readings with non-RMS meters are one way to tell how close your MSW inverter is to providing a quality waveform.
see
Discover Power FAQ or
AmplePower to get more confused.