Apart from normal inaccuracies that a unit might have, some units have an additional, dynamic accuracy. These units include mortars and howitzers.
You will see this when you order an attack command on an area or enemy, as one of three labels will be shown near the target area: "no sight", "targetting..." or "targetted". These refer to the three states of inaccuracy the unit can be in.
This state only appears when the target area is not being observed by any of your allied units, more commonly referred to as "no los". An artillery unit without any corrections will never improve it's aim between shots and will generally have very poor accuracy. This is not always bad, as sometimes you will want an area to be under fire at once, probably with a large amount of artillery installations, but that quite expencive and is usually avoided.
This is the least accurate state of the unit.
This label indicates that a unit can correct it's fire and is actually doing so, but has not reched it's maximum accuracy yet. An area or enemy with this label must have line of sight from an allied unit. This state has varying levels of accuracy, slowly improving over time. Mortars spend usually fire 1-2 times in this state, while howitzers fire 3-4 times.
The maximum level of acuracy for any unit. Once reached, only the movement of the enemy or redesignation of the attack area will reset the accuracy. Mortars do not miss at all in this state, while howitzers just reach a state when their innacuracies don't really matter anymore.