product quanity no subtracting when customer add 1 product to cart at a time

hi everyone i'm running opencart 2,1.0.2 journal 2 theme my problem is if a custom add a product to cart 1 at a time it allows them to check out even though my product quantity is set to 1 and to subtract https://prnt.sc/gx9kol I got option running for eg size etc I set them to not to subtract as the main one set the quantity to subract. (but if I select quantity in the quantity button 2 it say out of stock https://prnt.sc/gx9kol as I got stock set to 1 but why is it not doing this when customer add 1 at a time to cart ?? it the same product that they add just with different option selected
please help please see invoice https://prnt.sc/gxa49k so it would allow the customer to order unlimited amout doing it this way