I'm pretty sure it is just a question of tolerance on the system - just like thermostats etc, they have an 'operating range' before they either 'switch on' or 'off', so the speed keeping will never be precise. As linktrained says,. pax movement will be one possible cause of changed speed. I have seen a 737 classic 'fail' to throttle back until it would have hit the max mach without manual intervention. There I suspect a knackered A/T system, and of course we all know how rubbish the 737 speed control in descent can be.