Net carb confusion

Hi all, 

I've used the app to add my own food. 

One of my meals has 15g of carbs but 17g of fibre, therefore I assumed the net carbs would be 0 however it is showing as 15g. The app is then changing the total carbs to 32g so it is adding both values together. Can someone explain to me why this is? I've bought keto ready meals so I assumed they would be much lower in net carbs than this! 

Thanks in advance