Mono to Stereo - Signal Loss ?

I'm sure there is a perfectly good reason for this but.... try this:



Setup a Mono track with a Signal Generator (1k Sine @ -20dB).

Bus it to a Mono Aux (Send fader normalized).

The Aux loudness meter will read -20dB.

All good so far.



Now, do the same thing but bus the mono signal to a Stereo Aux.

The Stereo Aux loudness meter will read about -6dB less (-3dB per channel) than the original -20dB test signal that was sent.



If a Square wave is used it will still be -20dB.



How do you explain the attenuation happening when going from Mono to Stereo using a Sine wave in this situation?



Is this Black Magic or Science?