the electric test systems really don't test actual chemical levels- they test conductivity and such and "assume" the changes in conductivity is due to chlorine or salt or whatever.
This can lead to very accurate results when chlorine is only possible chemical that affects conductivity, but inaccurate when you add other things (like salt) that also affect conductivity.

Super-simplified, but that's ultimately why the Ort system has issues, at least part of the time.