Hi,
This could be because of packaging options used and the number of packages in the cart, different boxes might have been selected for different orders because of the number of packages that might not fit in the smaller box, But if the number of items in the cart is the same as the previous order with the same box packing option and you still get different boxes selected we’ll have to your site, we could check this issue better.
Regards,
Lorenzo
Team Xadapter