I don't think you're getting it.Hail of thorns has a miss effect also but I do not see as better for a single target upcast than Lighting Arrow. I will admit that situationally with multiple targets Lightening Arrow may be better.
Do not think it move the graph much.
You can only cast hail of thorns when you hit when an attack.
You can cast lightning Arrow anytime you hit or miss with an attack.
Lightning arrow is Graze with Lightning on a Bow.