Proverbs don't make sense

Mom is practically sprinting down E. 7th St.  At first it isn't clear, but she has two young children in tow, maybe nine years of age between them. They are loaded up: backpacks, jackets over one arm, lunch boxes, and the little girl somehow is clutching a half eaten apple in her left hand. The apple is mostly brown, as she and her brother are scrambling to keep up with Mom. One of them makes a comment, which is drown out by a passing truck, but ends with "...my apple!" Mom turns her head, and says only: "An apple a day keeps the doctor away!"

The girl gives the only reasonable response. Her nose crinkles up and she slows her pace, trying to digest, and she says: "What?"