this is because there are 3 sizes of hitboxes (small, medium and large). theyre shaped in a circle around your champion. something similar goes for skillshot projectiles.
the hitboxes dont have to match the shape of the champion model. in fact in many cases they dont. now imagine them also not matching the projectile perfectly.
skillshots are considered a "hit" if the circular skillshot hitbox touches the circular chamion hitbox on any side. this creates weird effects where skillshots hit even if they visually dont, because the invisible hitboxes are touching.
i dont know why we arent allowed to display champion hitboxes like in custom games. this would fix so many issues about how misleading they are.
i think the best way to explain it is to imagine league models like tabletop figurines. imagine theyre all standing on a circular platform, and that platform is the champions hitbox.
only due to the isometric view, the hitbox also appears mostly shifted behind the champion model.
https://www.reapermini.com/images/blurbs/savageworlds/2up.jpg