Atillery Indirect Fire

[Gordons HQ]

Can someone tell me why artillery is more effective the closer you move it towards the enemy.
I've often wondered over this point as a shell when landing surely would have the same explosive value whatever it's range.
The only thing I can come up with here is there would be less chance of the shells being affected with things like being blown off course by the wind. Perhaps also other things like wear to the barrel of the guns.
I don't fully understand this though because if the fire is observed then the artillery fire will hit the plotted 250m hex, it's only if unobserved it usually misses.
Any thoughts on this anyone.

Cheers,
Gordon