That's because DAFNE's done as a one-size-fits-all thing. It's basic logic. If you assume that food item A has the potential to raise your BG by 6mmol/l with no insulin, and that at 9am you need 5u to cover this, and then at 6pm you would need 10u, then logically the amount of insulin you need as a correction factor must change too.
I suspect if you raise this issue with a diabetes nurse they'll do the usual trick of telling to not be so silly and listen to them and their half a day worth of CPD they do each year, rather than people who have actual diabetes.