Why does Scout tell Atticus that she agrees with Mr. Tate?

The sheriff urges Atticus, this once, to accept the situation even if it's not perfect according to law: Mr. Ewell was responsible for Tom's death, and the sheriff urges Atticus to "let the dead bury the dead." He says that it would be a sin to drag shy Boo Radley out into the limelight, and declares officially that Mr. Ewell fell on his own knife. Atticus, deeply moved by this revelation, asks Scout if she understands. Scout assures him that she does, explaining that having it another way would be like shooting a mockingbird. Atticus looks at Scout with a sense of wonder, and thanks Boo for the lives of his children.