Shopify allows customers on your Shopify store to add more than what you have in stock.
For example, if you have only 10 of a product in stock then the customer can only add 10 to the basket when they first create the order.
However if you go into the basket, you are able to add more and click save to increase the total ordered. So if they added 5, then it would be 15 in total; despite the previous page saying you only have 10 in stock.
This is an issue on Shopify's side and they have the this Help Guide to help resolve the issue.