The NPK values are the percentage of the product you are using that is nitrogen, phosphorus and potassium. So if you had a product that was 2-4-6 then it would be only 2% nitrogen meaning if you added that to a product which contained no nitrogen then percentage of the new mix couldn't be 2% as the nitrogen levels would be diluted from the product which contained no nitrogen...The same principles would apply to the phosphorus and potassium values as well.