The reason for this is because when a player is moving quickly, with hitscan you normally want to leave your mouse on someone and stop it there, with a projectile you're leading someone's movement, and if that someone is moving fast e.g. in an airshot you're normally going to quickly move your mouse past where you think they're going to go.
Because you don't need to be pixel perfect or anything with projectiles, people often aim by flicking and click when they think the shot is going to be where the person goes. With a really fast movement like that, they often then overshoot and bring their mouse back to the position it was in. This becomes muscle memory after a while so it's why you see people flick over someone and then back after a shot lands.