AFAIK that's a technical limitation of the octamed classic 8-channel mode, which itself was derived from some earlier trackers.
The technique of playing more channels of music than the Amiga hardware was capable of was first introduced with Jochen Hippel's "Hippel 7V" routine, which used one hardware sound channel, and performed software mixing of two channels as the source of the remaining three Amiga hardware sound channels. The reason for using seven channels rather than eight was because the sound routine required more processing power than the 7.14MHz 68000 CPU in the older (and later low-end) Amiga models could provide. The seven-channel routine then appeared in TFMX. Finally, the routine was optimised so it could mix an additional channel, resulting in eight channels of sound. The 8-channel routine first appeared in another tracker called Oktalyzer and Face The Music. Finally, this appeared in OctaMED.
(from
wikipedia: Octamed)
Octamed SoundStudio (which does true multichannel mixing) doesn't have this problem.
