Hello, I have Red Sea test kits, when I test with one method of 5 ml of aquarium water as written on the test card, I get a result of 5 PPM.But if I check with the comparator that comes in the pro nitrate kit and check with 15 ml of osmosis water and another 1 ml of aquarium water and then multiply by 16 as written in the instructions, I get 24 PPM, how is this possible?