I believe it is a bug in the build. Global volume is essentially the master volume level for all the different sound types (music, voice, effects & movies).
global volume = 100; sound volume = 100 - the sound volume would be played at the loudest setting possible.
global volume = 80; sound volume = 100 - the sound would be played at 80% volume.
All sound channels take into account the global volume, the volume of the sound type & the volume set for the played sound file itself. Therefore it returns the calculated volume based on the calculated percentage returned from what I just mentioned.
here's a quick bit of math: (I hate math by the way)
global volume: 80%; sound volume: 70%
-- + --
70% of 80% = 56% (70 / 100 = 0.7 | 0.7 * 80 = 56)
I know this doesn't exactly provide a solution, but hopefully it explains how the volumes are calculated.