Back quivers were very rarely (if ever) used in the middle ages, instead the arrows were often carried in a leg quiver. Reason being; if you try to run (or worse, get on a horse, the arrows would simply bounce around and put of the quiver. Historically (though perhaps not in medieval Europe) archers held (some of) their arrows in their bow hand or their string hand, being able to fire extremely rapidly.
The back quiver is more of an Olympic/target archery item where you fire your arrows whilst standing still.
It’s also worth bearing in mind that arrows shouldn’t be so easily recovered, medieval broadhead arrows (except for hunting arrows) were very loosely fixed to the shaft (usually with wax or spit, often just before a battle), this would be so anyone trying to pull an arrow out by the shaft would get only the shaft of the arrow, the head would be left inside, often causing a slow and painful death from infection, I’m assuming these are the “wounding arrows” sold in-game.