Soldiers have perfect straight shots.
Demomen have grenades the size of large oranges that bounce around like rabbits and require PERFECT AIM to even hit a target.
L2Demo/Soldier.
Aim at the ground so the grenade bounces UP INTO THE PERSON'S FACE. Demomen and soldiers have mostly the same tactic of aiming lower.
Soldiers require aim as well. You can't aim directly at a person and expect to hit them with a rocket unless you're close enough. People know how to move out of the way, you know. So, you have to aim in their general direction and shoot at least 2 rockets.