It is not that arty fire is unable to hit the same spot twice...
it's just that the probability of that happening is very low.
Furthermore, the concept of arty fire is area target: 75m by
75m
But with modern technology, achieveing multiple hits for the same
target is now possible.
Arty rounds travels a great distance before reaching the targets.
For low charge firing, the distance is from 5 km onwards. The
FH2000 has the furtherest range of up to 40 km. Unlike firing a
rifle where u can see the target, arty battallions reply on FOs
(Foward Observers) to tell them the coordinates of the targets.
These FOs are also known as the Eyes.
As the range increases, the accuracy of the rounds will suffer.
This is becos the shells are subjected to various weather
conditions such as wind, density of air .... etc. Even the rotation
of the earth and gravitational pull plays a part too. The weight of
the shell is a factor too and often, the shells used are of
different weight,. Other factors will be the accuracy of the
coordinates of targets as FOs usually rely on visual
spotting.
Usually for Arty, the FO will use a reference point as the initial
target. He will do ranging (ie: zeroing) where he will move the
rounds thru various corrections to the reference point. Once the
arty guns are "zeroed", he will then have then call for fire on the
"real target". This will increase the accuracy and ensure maximum
devastation.
I was lucky to be in arty but was even more fortunate to be a
Gunnery Spec. Unlike the Arty Specs whom are the gun commanders, GS
do not learn how to operate the guns. But we learn about the
science and physics of arty. Believe it or not, one of our SOP
equipment is a scientific calculator.....
Hope my answer clears up some doubts which you may have! 
