As arrows are projectiles, they must use the (large unrotating unintuitive) physics hitbox instead of the bullet hitboxes. So how does the game know where the head is? Is it the same for all classes? How large is the headshot area?
This has been a topic that cropped up a lot in the past its very hard to discribe best to show:
here is the jist of the way it works:
projectile and hit-scan hitboxes are boxes around your character. if a shot hits it, it's a hit. Same with headshots from arrows. It's just the hitbox for huntsman headshots is quite large, otherwise it'd be near impossible to get headshots with it.
in broad terms the radius to hit a headshot with the huntsman is larger than the standard sniper rifle due to the weapon being projectile based rather than hitscan.
Related look up hitboxes in tf2 / wireframes of models and you will be able to see what they look like. Hope this helps. link to video about hitboxes (each class)