Hanzo's arrows can be funky. Sometimes it looks like he missed, but he hits anyway. This doesn't truly matter that much in the grand scheme of things, until you're watching a Play of the Game where the arrow is completely off-mark and you're glaring at the screen for an undeserved kill.
YouTuber Vultcha looked up how easy it is for the Shimata to score a headshot, but ended up going down a rabbit hole where he investigated the hitboxes for most of the cast, and most of their weapons.
There's a notable difference between 'hitscan' and 'projectile' weapons. Hitscan means you click, and if you're on target, you hit. The bullet you fire is just for show, you've already hit the target, it's just a special effect between you and the enemy. Examples of this are McCree's revolver and Tracer's pistols. Unlike those weapons, projectiles like the sonic waves from Lucio, shuriken from Genji, and arrows from Hanzo, have to find out if they hit the enemy. And it seems like the hitbox is a little bit more generous for those projectile users.
The video at the header analyzes Hanzo's arrows on the cast. You can check out Vultcha's second video below, where he checks out the weapons of the rest of the characters.