Stereo quieter than mono

[K-Dubber]

Sorry for all the questions! So whilst trying a stereo iem mix (linking 2 aux channels together) I noticed that the overall volume is considerably quieter than when my transmitter is set to mono mode. Is this the usual case and some sort of scientific reason that I should already know about? 

[sam.spoons]

if mono is a sum of left and right you'd expect it to be between 3 and 6dB louder (two mono signals summed would be +6dB, two unique signals summed would render +3dB). That would account for a noticeable increase in level (and is also the reason for pan pots having a level drop of, usually, 4.5dB when panned centrally).

[K-Dubber]

Great, thanks! Now to sort out the interference I'm getting in stereo mode!!