One reason is that the system assumes the soil is a blank...

One reason is that the system assumes the soil is a blank medium and devoid of natural nitrogen. We know that's not true. And the problem we run into is that we don't know exactly how much nitrogen is in the soil and how much will be available to the crop. The release of nitrogen is dependent on the weather, so there's always a possibility of adding more or less nitrogen to the soil than is needed.