atry not on target

[Colonel Talvela]

Here's what most likely happened in that situation.
The "spotting round" (before fire for effect is called in) has landed out side the spotters view. Therefore the art. observer was unable to call in a correction. The result is a random barrage, etc..

I am pretty sure it doesnt have anything to do with where the spotting round lands. It has to do with the FO's LOS to the target spot the instant the spotting round lands.

much more details here - battlefront discussion