I don't think hanna being off by one is a known thing or even that common. But it is something to keep in mind.
If you really want to know wich one is closer, get a bottle of calibration solution and take a measurement with both tests at the same time. But then again I've seen a post that someone's calibration solution was off.
I my case my trident calibrated with there solution measured a completly different bottle of solution spot on while my hanna measured it 1 Dkh higher. And is still the case with other bottles of calibration fluid.
If you really want to know wich one is closer, get a bottle of calibration solution and take a measurement with both tests at the same time. But then again I've seen a post that someone's calibration solution was off.
I my case my trident calibrated with there solution measured a completly different bottle of solution spot on while my hanna measured it 1 Dkh higher. And is still the case with other bottles of calibration fluid.