Apple Pie 4th of July

I'm not sure what this book is trying to say. A Chinese-American girl tries to convince her immigrant parents, who run a Chinese restaurant, that nobody wants to buy Chinese food on July 4. And for most of the day, she's right. Until 5pm, after the food made earlier was no longer good, when a bunch of customers come and the parents mysteriously have just made more.

What does this mean? Is it just pointing out that Americans don't get takeout for lunch on July 4th? What does the normal customer flow at the restaurant look like? I have no frame of reference here. Is the girl wrong? Right? It says that her father has lived in the US since at 12; shouldn't he know about July 4th? How long has this restaurant been open? Do they have at least a previous year of experience?

It's almost like the protagonist is saying, "My parents can't understand this, they're not American like me," so why do they make the second batch of food? What is going on there? This is just baffling. Is it supposed to be a metaphor for America not seeming inclusive but it actually is?