Accuracy
Accuracy is one of the factors, that determines how much damage your troops actually do in a battle: their numbers in your army and their attack power according to the Encyclopedia is only the starting point... all other effects reduce that amount to the actual amount subtracted from Health points and determining the casualties. Accuracy is one of them.
Normal units have 30% (base) accuracy in regular attacks, and 20% (Siege-10%) accuracy in siege attacks.