It's counter-intuitive that the PH will rise, but it doesn't detract from my point that parameters have changed and the test my not be designed to work accurately under those parameters.Adding totally pure fresh water (at pH 7.0 ) to seawater (at, say, pH 8.1) raises the pH above pH 8.1 for well understand, if not intuitive reasons.
The reason is that bicarbonate is a stronger acid in seawater than in fresh water. Adding fresh water makes it a weaker acid, and raises the pH by this reaction:
CO3-- + H+ --> HCO3-