pH and ORP sensors look alike but have different measurement principles. pH sensors rely on a glass membrane that produces a voltage signal in relation to the hydrogen ion concentration in the liquid. The concentration of acids or bases affect this hydrogen ion concentration and thus influences the pH measurement.
ORP sensors measure of the overall level of oxidizing or reducing substances present in the liquid. The ORP measurement is not specific to any single ion in the liquid thus the response is based on the net presence of these chemicals.