hanna checker po4 ulr vs colombo and salifert kits

HI, Im using the Hanna Checker Phosphate ULR for a while now and im not sure if it gives the correct reading. Today i did a triple-check and got the following readings: Hanna: 0.7 (first reading was 0.8) Colombo: 0.0 (double check) Salifert: 0.0 (double check) Not sure what to trust here...