Harry had no way of knowing exactly how the rooftop chase would play out
If Harry had deliberately zapped Hermione to make her stumble and distract Draco, that would explain everything, but in the text he's running away, dodging their spells. He might have set some sort of trap on the roof, but there's no indication in the text.
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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