I have a Lamotte Digital Tester for Salt and its way off after only one season.

I believe an electric current is passed between two points for this test and the points can become corroded or it can just become off over time and need calibration.

The strip test is usually off for me and hard to read anyways. When I knew how much salt I had in a new fill the strip showed far more salt than was actually in the pool.....the drop test was spot on.

I would trust the drop test. Just be exact with your measurements.