I'm not sure we fully understand your question... however, I'm guessing its to do with what's more important to the autopilot... for example the 75/6 will stay straight and level 99.9% of the time however the speed can vary + or - 0.03Mach. We a accept that as part if the norm, I believe the ABus is the other way round sacrificing a few feet either way to keep the speed bang on... ??
Cheers..
DC