Heroism is a question of character, not just results. For example, an assassin who gets cold feet when he sees his target's bodyguards is not as heroic as a police officer who takes a bullet to save the assassin's victim, even though both decisions save the person's life.
It requires a greater degree of selflessness to value two lives over your own than it does to value a hundred lives over your own. A fairly large number of people would consider sacrificing themselves to save half the world — that many lives lost is such a tremendous tragedy, it almost seems like an obligation. But few people would seriously think about exchanging their life for a stranger's — they'd say sorry, too bad, you got unlucky.