Well you are probably just feeling the water temp right where you are adding the water. Here is the math. When you evaporate one gallon of water the pool loses 8500 BTU of energy. When you replace a gallon of 90 degree water with a gallon of 60 degree tap water you are cooling the pool by about 250 BTU of energy. Thus, the one gallon of evaporative loss is equivalent to about 34 gallons of cold water added.