Na verdade, vou tornar o meu comentário uma resposta.
De acordo com J.K. Rowling, a diferença entre a morte de James e a de Lily é que Voldemort sempre pretendeu matar James; Ele deu a Lily múltiplas chances de se afastar e deixar Voldemort matar Harry, e ela se recusou e morreu protegendo Harry. Foi isso que criou o encanto protetor entre Lily e Harry. Lily escolheu morrer por Harry; James não teve escolha.
ES: This is one of my burning questions since the third book - why did Voldemort offer Lily so many chances to live? Would he actually have let her live?
JKR: [silence] Can't tell you. But he did offer; you're absolutely right. Don't you want to ask me why James's death didn't protect Lily and Harry? There's your answer - you've just answered your own question - because she could have lived - and chose to die. James was going to be killed anyway. Do you see what I mean? I'm not saying James wasn't ready to; he died trying to protect his family, but he was going to be murdered anyway. He had no - he wasn't given a choice, so he rushed into it in a kind of animal way. I think there are distinctions in courage. James was immensely brave. But the caliber of Lily's bravery was, I think in this instance, higher because she could have saved herself. Now any mother, any normal mother would have done what Lily did. So in that sense, her courage too was of an animal quality but she was given time to choose. James wasn't. It's like an intruder entering your house, isn't it? You would instinctively rush them. But if in cold blood you were told, "Get out of the way," you know, what would you do? I mean, I don't think any mother would stand aside from their child. But does that answer it? She did very consciously lay down her life. She had a clear choice.
ES: And James didn't.
JKR: Did he clearly die to try and protect Harry specifically given a clear choice? No. It's a subtle distinction and there's slightly more to it than that but that's most of the answer.
INTERVIEW WITH J.K. ROWLING - 07.16.08 - MUGGLENET