You didn’t do anything wrong. Different methods of measuring salt gets different results. All the measurements are not very precise. As long as your SWG is happy and generating CL then your salt level is good.
If your SWG says low salt then use your test kit. If your test kit says salt level is good then your cell or flow switch thermistor is going bad.
Salt tests are not particularly accurate. Most are +/- 400 ppm. Salt tests are just to get you into the ballpark and check your swg reading against some other reality. Once the SWG is happy and making chlorine then life is good. The discrepancy is likely due to lower water temp. The 1766 is widely thought to be most accurate and not impacted by water temp. Pentair and most SWGs measure conductivity and not salinity which is impacted by water temps lower than some where around 80. Lower water temps read as lower salt levels.
The IC40 is known to not be the most reliable when testing salinity. +/- 500. There is a thermister in the Flow Switch assembly that often goes bad causing erroneous salt readings.
As Marty points out, the key is to keep the cell happy until the actual salt level gets above 4000 or so... As long as the cell is working, and not reporting an error, I don't care what the cell reports. When I have to increase the salt level above 3800 - 4000 range, to keep the cell working, then it is time to replace the Flow Switch.
BTW, the IC40 will report a high salt error when it thinks salt is above 4500 but it will continue to make chlorine. I raised my salt level well above 5000 the first spring because the SWG kept saying low salt and I kept adding more. It worked fine all summer and I didn't have to add any salt for a long time.
Just to be clear.. My recommendation to not have the actual salt level go above 4K is only because I find that I can start tasting it more than I like.. I agree with dv, that the cell will work fine even with the high salt light on..