Contrast measuring a Stereo Track?

Contrast will measure stereo tracks.

I have two mono tracks, one twice the volume of the other.

-14.8dB
-20.8dB

I make them into stereo and measure it. The result is -16.48dB, not -17.8dB I was expecting.

I have two identical mono tracks at -14.8dB. I make them into a stereo track and it measures at -14.8dB. Simple divide by 2.

I have two mono tracks, one at -14.8dB and the other silence. Make them into stereo and the new value is -17.8dB.

3dB drop?? Not 6dB?

Koz