What is ironic about Marjane's mother commenting to the lady at the supermarket?

In the section entitled, The Jewels, Marjane's mother admonishes a woman about fighting over food, telling her that if people only bought what they needed there wouldn't be a shortage. Ironically, after they leave the store, Marjane's mother tells her they need to hurry over to the convenient store and buy as much rice as possible.... just in case.