3rd Person view has some pretty ridiculous aiming issues.
If you stand about 2-4 meters in front of a person and throw a grenade at their torso/head, the grenade will fly over them.
If you do this in first person, it hits.
If you do this in third person after moving them out of the way of your crosshairs, the grenades' path changes and it travels through their previous position.
This also applies to all other weapons, just more noticeable and easily reproducible with grenades.
Additionally, you can hit people who are directly behind you while in 3rd person view.