When you execute a delivery line, it calculates whether the sum of all other goods delivered and reserved is with the quantity about to be saved, yet within tolerance. If so, then then amount within tolerance is marked as not used unless you un-tick the write-off box to do so.
So if you follow a cycle of reserve 1 => deliver => reservation 2 => deliver => reservation 3 => deliver
then it will on every delivery move the deviation back into the open quantity except on the last delivery.
But if you follow a cycle reserve => reserve => reserve => deliver => deliver => deliver
then since there is no open quantity it will by default not move the under-delivery back, to avoid tiny open quantities building up.