Hmmm. sounds like TDS might be at play here. High TDS will increase the electrical conductivity of the water. Ideally the TDS should be almost the same as the salt level from what I have been able to ascertain. Not familair with the SWG you mention so I don't know if it has a readout on it of salt level which is usually a conductivity reading of the water but the introduction of nitrates and phospates from fertilizer runoff would certainly raise your TDS and that would produce a correct reading with a much lower actual salt concentration in the water.
Retired pool store and commercial pool maintenance guy.
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