Shopify allows customers on your Shopify store to add more than what you have in stock.
For example, if you have only 2 of a product in stock then the customer can only add 2 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 7 in total; despite the previous page saying you only have 2 in stock.
This is an issue on Shopify's side and they have provided the below help guide to help resolve the issue: