barbaros
Knight at Arms

Well, I may be wrong because I didn'perform horse archery in real life but here is my suggestion. When aiming a bow from horseback, targeting reticule should make small periodic vertical movements in synchronisation with the horse's steps. Those movements can reduce to a minimum when a character reaches very high horse archery skill. A novice horse archer should not be able to stabilize his aim perfectly when the horse causes vertical vibrations in each step.
To make an engineering analogy, today's main battle tanks use sophisticated technology to stabilize its gun vertically (lock it to a target) when moving on a rough terrain. Compensating the vibrations caused by the terrain (or horse steps) is very complex job. Before this (during WW2), tanks were firing only after complete stop. I know machines need high tech to do very simple tasks (for a human) like climbing stairs or running properly. A human can do those things without technology but only after certain skill and practise level.
To make an engineering analogy, today's main battle tanks use sophisticated technology to stabilize its gun vertically (lock it to a target) when moving on a rough terrain. Compensating the vibrations caused by the terrain (or horse steps) is very complex job. Before this (during WW2), tanks were firing only after complete stop. I know machines need high tech to do very simple tasks (for a human) like climbing stairs or running properly. A human can do those things without technology but only after certain skill and practise level.