Hmm... say you estimate that you will sell 1000 items of "normal size", you stock 1000 items, and hope that you sell all of them. You end up selling 900, you have a remaining 10%.
No say you estimate that you will sell 10 items of "less common size", you stock 10 items, and hope that you sell all of them. You end up selling 9, you have a remaining 10%.
How does that make a difference?
There is no global definition of "less common size". It varies greatly from one locale to another. At the same time, production has relatively high fixed costs and is centralized.
It would be very expensive for the global factory to customize the distribution of sizes manufactured for a retail store in Des Moines, Iowa. The order is tiny and it would require customized logistics, all of which greatly increases cost and complexity.
It's more like if you find one mushroom in the forest, it doesn't make sense to bring it home, get the knife, clean it, get the pan, oil the pan, fry the mushroom, eat it, clean the knife, clean the pan, put things away. It's not worth the effort for just one mushroom. If there are many, a lot of these actions only need to be taken once.