I'm confused. Not by your son's initial answer, which is obviously correct, but by the teacher's. However, it is late and I am sick with a cold right now.
Here is my suggestion: get him to ask or email the teacher with the simple question, "I don't understand. Can you explain? I checked my answer and I know it's right and you marked it correct, and I don't understand why I should do this additional step" (or whatever it is; cold-addled brain not functioning).
That way, she'll have to show her reasoning. If it doesn't work, she, err, won't be able to. Email won't put her on the spot (though she might ignore it). There is always a risk that she'll get angry if a student finds that she made a mistake. But my brain is addled right now. Maybe someone else will understand this better than I can at this point.
