The dimensions we provide for each of our dry bags are the exterior max dimensions of the product to help give a reference of the overall size of the product. The liters equal the carrying capacity Volume and are measured using ASTM F2153-07(2018) standard.
Giving flat measurements for our bags is not an accurate depiction of their volume. We measure all our bags by volume and we calculate the volume by how much water the bags can hold.
Here is a video of us measuring the capacity of a Reckless 40 for a visual of reference:
This is very common in the luggage/travel markets when referencing products.