Simulation first estimate the body of the box with mesh which will not be 100% if there is any curve, circle, arc.
Hence result is not 100%.
I‘d suggest increasing your mesh to see if that affects the accuracy.
If possible perform a mesh sensitivity study.
Here is a link to my website where I detail the process I use for Mesh Sensitivity Studies , hope it helps.
I may do that when I have some spare time. Thank you for the suggestion.