From Hanna's website it states the accuracy of the phosphate checker is + or - 0.04ppm. So even if you read 0 the machine is still working in range if you really have 0.04ppm. If you got a reading of 0.04ppm you could really have 0ppm or 0.08ppm and the checker is working properly. PPM/3 * 1000 gives pretty close to ppb the phosphorus test is reading. So the machine's built in accuracy is around 13ppb plus or minus.
The phosphorous test is + or - 5ppb. So your reading of 16 could be 11 or 21 and the checker is still working ok.
So your reading of 0ppm and 16ppb is still in both checkers range of accuracy.