Hallo Andrew,

average value of pure sinus is of course zero. Your factor 0.636 comes from something different.

Assume you want to measure RMS value of sinus. How to proceed?
Today you choose a RMS-to-DC-converter, like AD636, or similar. But in fomer times RMS was measured differently: First, sinus was full wave rectified and afterwards low pass filtered. What you get is MAD (mean absolute deviation) or 'ac average'. For a sinus MAD = 2 / PI x 'amplitude' = 0.6366197... x 'amplitude' = 0.637 x 'amplitude'.
But RMS is 'amplitude' / SQRT ( 2 ) = 0.707 x 'amplitude'. So, reading of MAD-meter must be corrected by factor 0.707 / 0.637 = 1.111 for achieving RMS-value.
This factor you will be always faced to, when dealing with simplified RMS measuring. Unfortunately, this factor is only correct for sinus, but was widely used for other signals too, in the past, as long as RMS-to-DC-converters were not abvaible.

Bye,
Kai