That’s how casualties are counted. A casualty is someone who’s no longer able to fight, because they’re either dead or badly enough injured that that can’t get immediately patched up and sent back into combat. Either way, the effective manpower is diminished.

If you’re fighting an enemy that’s not completely inhuman, it’s actually more effective to cause lots of injuries to them rather than killing them outright, since then they’ll devote resources to saving their injured comrades, evacuating them, giving them medical care, etc., all of which diverts manpower, logistics and money from their main job of trying to kill you.