Blast damage has unlimited vertical range
The explosions of rockets and barrels only check the horizontal position of an object when determining how much damage to do to objects within their blast radius. As a result, the pattern of damage is a cylinder of infinite height rather than a sphere. The effect is most noticeable when firing a rocket against a wall high above a monster or player. The victim will be injured or killed because it is within the horizontal blast area, despite standing far below the point of impact.
This situation has similar roots to solid objects making the space above and below them solid, and the ability of monster's melee attacks to hit at infinite vertical range.