FWIW, I checked the code and it would appear that you are not entirely correct. The cached mall price is actually the price of fifth *item* you can buy, meaning that if somebody has an offer at minimum price without limit (or limit of at least 5) (and 5+ available), you will see that price, even if the next price is much higher.
At least that's the logic in StoreManager, but I didn't find any other.