But the side it turned to was the side necessary to hit its original target. Given that spells have some sort of natural AI (like the enchantment on Harry's pouch), and that basic wanded magic is just a matter of triggering a pre-defined effect by saying an incantation while moving in the right way, it seems more likely that the spell was intelligent enough to change direction to hit its target than that Harry pre-programmed it with that specific directional change during casting.