I was always of the opinion that artillery in SP spread out too much when impacting, even with a FOO, however I have recently read a book that reviewed the Canadians in Normandy and what they found (this study was done in Oct44) is that less then 50% of the rounds fired landed in the predicted impact zone, and less then 5% actually landed in a 100x100 foot target area. The vast majority of rounds were off for both line and depth.
So I will stop saying WTF when my barrage lands 6 hexes away.