It might happen that a battle ends in one round or the losses are higher than 50%, with more than 100 (500 in Fortresses and 2500 in Cities) units on the attackers side, lets take this case for instance

The attacker sends 500 Crossbowmen to the castle
The defender has 60 Armoured horsemen and fortifications on level 20

The result would be the loss of the 500 Crossbowmen on attackers side as well as a loss of 80 percent of the defenders units. So this rule is bound to the combination of units on both sides.