Why do products with quantities set to "Exact" still sometimes get oversold?

The stop at zero function only prevents further orders from being placed once the item is sold out. It does not prevent the last order from going over.

The customer is told twice along the way, including once in bold red letters, that they might not get their complete order if they're trying to order more than is marked available. It would not be a surprise for them, if they ordered two when the quantity available was only one, if they did not get both.

As for the growers, I've found that often times they actually can fill that last order (and get that extra bit of money) but if not, they can very easily remove what they can't fill with a single click on their open orders page.