This just means the seller is intentionally limiting how many they are willing to sell to a single customer in one transaction. 

Since most inventory tools use the 999 trick and add the items to the cart this is the amount they are showing as "in stock" when in reality it could be significantly different. 

This is why we implemented the limit buster functionality to "see" past these limits for FBA sellers on

Please see the video below that shows this in action