What really matters is what the unit is telling you because if it doesn't think the salt level is correct, it will shut down. So adjust the levels to keep the unit happy and produce chlorine.

However, cells will tend to read low when they are nearing end of life so the difference between what the unit reads and actual salt measurements are important but the other tests need to be reliable and pool store tests are not always reliable.

Also, I found the Goldline salt readout to be fairly accurate and nearly always will match my Taylor salt drop kit within +- 100 ppm. However, I have had problems with salt strips in the past and they tend to read high plus they can easily go bad if not stored properly so I no longer use those. Also, if the pool store used a TDS meter, those need to be calibrated to read accurately. So unles the pool store used a drop, I wouldn't trust the results.