Hi Sceadwian,
That could be because of several reasons, one being the increment time and
another one could be that it just has too much error in the calculation
somewhere. You could try a smaller increment time and see if it gets closer
to 120v.
For the OP:
RMS is a statement about the power producing ability of a waveform...
it is a measure of the effectiveness of a source in delivering power to
a resistive load.
It stands for "Root of the Mean of the Square", hence the "RMS".
To calculate this for arbitrary waveform, we first square the wave,
then compute the mean of that result, then take the square root.
Mathematically for a voltage wave it looks like this:
VRMS=sqrt((1/T)*integral[0 to T](v(t)^2 dt))
where
v(t) is the voltage wave function (and we squared it)
T is the period
and we used the mean value theorem to calculate the mean
and finally we took the square root of that result.
Now if we defined v(t) to be a sinusoid, we would end up with
a simpler formula that would be valid for sinusoids which happens
to be Vpk/sqrt(2).