that is how the program is designed. this is already in the ideas category.
but i am not sure how this would help in any way if it is implemented. for example, if you deliver 25.5KG of Item1, 10.3Litres of Item2 and 7Numbers of Item3 on a single delivery note, the total would show 42.8. i wonder how that would be useful since you cannot identify the item nor the UOM.