The 120VAC voltage coming out of the wall is not very accurate – for example in our office it measures 108VAC. It’s not terribly unusual for a 120VAC wall socket to measure 130V, I wouldn’t worry about that.
Regarding sample buffer depth and frequency settings: The Mooshimeter works by gathering a bunch of samples in a buffer, then calculating the RMS value of the buffer. When you lower the sampling frequency, you end up smoothing out higher frequency signals which may be present on the power line, which decreases the calculated RMS. If you lower the sample frequency to the extreme, you end up smoothing out the power waveform itself (it’s a 60Hz waveform, if you sample at 125Hz it will not give you an accurate RMS value).
The default settings (4000Hz, 256 samples) represent good settings for capturing RMS components up to 1kHz. You should not have to mess with the sampling settings unless you have a very strange use case.
Does this answer your questions?