The short version first: voltage regulation produces a "wavy" (rather than flat) voltage; Phil Busardo shows it in many of his
reviews on youtube when he reviews regulated mods. The peaks of that signal exceed your set voltage but in between the device is basically off, so "on average" it provides the voltage you requested. This on/off signal varies very rapidly so you don't really realize that it is not flat (for the most part).
Now the point is that in "mean" mode those peaks are higher than in RMS mode. In other words, it fires harder. That may burn your cartomizer (or clearomizer coil) so people usually recommend setting it to RMS mode.
Now the technical part: the main question is how to determine the "average" voltage. Say you have a harmonic wave U(t) = Umax sin(t); here U(t) is the voltage at time t and Umax is the peak value. If you take the absolute value |U(t)| and average that over t=0...2pi the result is <U>(mean) = Umax * 2/pi ~ 0.64 Umax. This is supposed to be your "set voltage" so the device determines the required peak voltage to be Umax = Uset/0.64; for example, if Uset = 4V that would give Umax = ~ 6.25V.
The other common way of averaging the wave is to first square the signal, average it, then take the square root (aka Root Mean Square). This gives <U>(RMS) = Umax / sqrt(2) = 0.71 Umax. Again, this is supposed to give your "set voltage" so the device adjusts Umax = Uset/0.71, which is less than for the "mean" mode above: for Uset = 4V you get Umax = ~ 5.6V.
Sorry for all the details, if I didn't explain well, just skip the last two paragraphs
