My guess would be that the filter media in the fridge had grown itself a colony of beneficial bacteria, which was converting Ammonia to Nitrates;
Also, dissolved CO2 will lower the pH of the water, maybe try taking samples of water from teh fridge and tap, leaving them out for a day and then testing, Chloring and CO2 should gas off at that point (Chloramine will not though)
Also, dissolved CO2 will lower the pH of the water, maybe try taking samples of water from teh fridge and tap, leaving them out for a day and then testing, Chloring and CO2 should gas off at that point (Chloramine will not though)