Because the teacher is wrong and it’s an idiotic question.

The question asks the child to explain how Marty ate more pizza than Louis. “He didn’t” is not an appropriate answer to that question.

We know that Marty and Louis didn’t eat from the same pizza, because Marty ate 4/6 of a pizza and Louis ate 5/6 of a pizza. We also know that Marty did eat more, because it’s right there in the question.

The only logical answer is that Marty’s pizza is bigger, and so 4/6 of his pizza amounts to more pizza than 5/6 of Louis’s smaller pizza.

The question should have been “Marty ate 4/6 of a pizza and Louis ate 5/6 of a pizza. Explain who ate more pizza.”