That depends on a lot of factors as well, a lot of fast food isn't made to order and some can be created ahead of time if you're expecting a lot of orders to come in. Fries, burger patties, some other fried goods like chicken fingers can be held for a little while without them going bad. There's always the chance that the people working the kitchen may have had the smaller order on hand but needed to make some fresh things for the larger order.