These things are not lab-grade instruments. There will a margin or error.
If you calibrated to 0 using RO/DI, you will likely have an error at 35 ppt.
If you calibrated at 35 ppt using reference solution, you will likely have an error at 0 ppt. It may even read less than zero.
IMO, By calibrating to the 35 ppt solution, you get the most accuracy at the target salinity. As you vary from the reference point, accuracy decreases.