This explains a lot. Thank you.Because the measuring method itself is imperfect and changes a lot with temperature.
Conductivity of 35 ppt seawater changes a lot with temperature. If you are trying to use the conductivity to determine salinity, you need to correct for that.
The refractive index of 35 ppt seawater changes a lot with temperature. If you are trying to use the refractive index to determine salinity, you need to correct for that.
The density of 35 ppt seawater changes a lot with temperature. If you are trying to use the density to determine salinity, you need to correct for that. FWIW, a hydrometer actually measures/responds to density, even if it reports in other units.
My tester seems accurate at 25C. I guess +/- 0.5ppt isn't that important, as long is it is consistent.