What would be the reason that the readings are different.
Because they different tests. Some test strips are a sodium only chemical test, your unit uses electricity to measure resistance/conductance and then "guesstimates" salinity from that, with sodium being only a part of the Total Dissolved Solids (TDS.) Titration can be regarded as more accurate, but I doubt that you are getting that test from a store for free. If a store is using a hand held meter to test, ask them "When was the last time it was standardized/calibrated?"
In the end, it really doesn't matter though. What matters is "Will my unit operate at the level it senses?" It is probably capable of operating on a much wider range, but limits need to be set, don't they? Do you know what the limits are? Probably can't adjust them, so it doesn't really matter either.
Say the pool stores are right , and you are at 2500. If it senses 3200, and works, you are good to go!(with 28% error.) If it reads 2500 and stops working due to "Low Salt" then you need to add salt for it to work (but you have 0% error! :-D) Would you rather have a working unit, or an accurate one?
When DH says that she is cold, the furnace comes on no matter what temperature the thermostat says the house is! Why disagree with her, or your unit, just keep them happy and life is easier!
just realized that I missed that in your original post.
