For what reason does Atticus believe Scout deserves a spanking even after Jack admits he made a mistake?

Atticus says that Scout occasionally needs correction. He does not mention spanking her..... Jack says he never lays a finger on her.

Scout’s got to learn to keep her head and learn soon, with what’s in store for her these next few months. She’s coming along, though. Jem’s getting older and she follows his example a good bit now. All she needs is assistance sometimes.”

“Atticus, you’ve never laid a hand on her.”

“I admit that. So far I’ve been able to get by with threats. Jack, she minds me as well as she can. Doesn’t come up to scratch half the time, but she tries.” “That’s not the answer,” said Uncle Jack. “No, the answer is she knows I know she tries. That’s what makes the difference. What bothers me is that she and Jem will have to absorb some ugly things pretty soon. I’m not worried about Jem keeping his head, but Scout’d just as soon jump on someone as look at him if her pride’s at stake…”