the difference stems from the physical definitions of fine and gross motor control.
juggling, among other skill sets that can primarily be analyzed with composite body groups, is a gross motor activity that is improved by understanding macro-dynamic and macro-energetic muscle motion. proper execution comes from understanding general manipulation of the body group rather than its components.
in the context of computer games, aiming, regardless of sensitivity & state of control (consciousness vs subconsciousness), requires much more precise hand-eye coordination & ultimately requires much more fine motor control than gross motor control during most instants in the motion. one of the biggest factors that humans have to differentiate themselves from other mammals is fine motor control; we allocate most of our mental resources towards its mastery.
with that said, aiming is a complex subject; aiming in the context of computer gaming is analyzed differently from how jugglers' aim. when you discuss the physical concept of aim, you have to rationalize the scope of the activity & the ability to discuss the topic using composite body groups versus individual bodies. in the case of gaming, you're not simply manipulating a composite body; you have to look at each subsystem and identify each point of failure (energy leech). as such, you would have to consider the various forces interacting with the mouse & the energy of the mouse-mousepad system, hand-mouse system, and arm-hand system. as a tip, one concept that might be tripping you up is the conservation of energy & its consequences; you have to account for nonconservative forces (friction) leeching energy from the conservative forces in the entire system.
the main point i'm trying to make is that there are various (non-constant) nonconservative forces leeching energy from the system (friction from the mousepad on the mouse) and/or altering the motion in some fashion. your goal is to identify those forces and minimize their effects (a few examples are friction, mouse stability (which may be better analyzed through the computer science and electrical engineering disciplines), hand stability, finger stability, arm stability, elbow stability, etc). unfortunately, given our biological constraints, fine motor control has too many uncertainties to be perfectly stabilized. to improve your aim, work towards approaching equilibrium, but don't expect to actually reach it.
biophysics is a pretty neat field; both small-scale (fine motor control) and large-scale (gross motor control) phenomena can't be adequately explained with kinematics. it can get pretty complicated!