From what I gather, the GBA doesn't have real channels like a regular APU. All the sounds are created via software other than the 4 GB/C channels instead of the sounds being explicitly restricted by the APU. The reason there are 2 other channels are for stereo sound. Most of your GSFs are mono with no use of the GB/C channels as that's what most GBA games were in general.
The Z80 does all the audio work for GBA and GB/C audio. I also believe that any "channel" limit is strictly the result of the Z80s power, so anything other than the normal GB/C 4 are all lumped together into L and R. More channels could be possible, but it would result in a significant hit to quality and require far more effort to program than most are willing to do.
Of course, I could be WAY off base, but that is what I have inferred from your observations, my knowledge of the hardware and my experience with writing GBA music.