What is Scout’s explanation for the beating she has given Walter Cunningham?

From the text:

Catching Walter Cunningham in the schoolyard gave me some pleasure, but when I was rubbing his nose in the dirt Jem came by and told me to stop. “You’re bigger’n he is,” he said.
“He’s as old as you, nearly,” I said. “He made me start off on the wrong foot.”
“Let him go, Scout. Why?”
“He didn’t have any lunch,” I said, and explained my involvement in Walter’s dietary affairs.