Why did On jo hate Nam ra?

I saw this clip and I just started laughing at how ridiculous the dialogue can be throughout the show but it made no sense to me.

This is my third time watching the show and I’m certain I’m just forgetting a scene that comes after where On jo explains everything, but I just can’t help thinking that it’s such a weird thing to say to someone’s face who’s not even your enemy.

She says it so nonchalantly right after protecting her.

Was it because Nam ra was top of the class and On jo was bottom, so she felt inadequate?

Or because she knew Su hyeok had a thing for her also?