Scout tells Jack that he needs to first of all listen to both sides of every story when there is a dispute between children, and second expect a literal reaction when a literal rule is given:
"Well, in the first place you never stopped to gimme a chance to tell you my side of it - you just lit right into me. When Jem an' I fuss Atticus doesn't ever just listen to Jem's side of it, he hears mine too, an, in the second place you told me never to use words like that except in extreme provocation, and Francis provocated me enough to knock his block off-"
At Scout's age she believed it was legitimate for her to follow rules exactly as she had been told them. She further believed that Francis' designation of calling her father a nigger-lover was justifiable provocation for a response. Although her response was inappropriate, she felt she was within the rules.
Uncle Jack had previously not understood that children don't read sarcasm or appropriateness very well. Children need guidance in order for punishment to work.