I realise that in matters relating to Warhammer it is heresy to try to logically apply real life situations to events, but...
I have noticed that some people when trying to rationalise the changes to the empire mortar - shouldn't that be howitzer? - have wondered why an exploding shrapnel shell causes less damage to mass infantry than a large stone the same body of men.
The answer is pretty simple.
The impact of the stone creates a storm of teeth and bones that explode laterally within the unit.
Add to this that prior to artillery developements of the late 19th century the use of shrapnel was a highly specialised and rather hit or miss affair; with the shrapnel spraying in all directions.