That variation suggests that the water source is subject to significant influence from rainfall or surface water. Since you are on Long Island, a terminal moraine with little area for surface reservoirs, I'm betting that the water source is groundwater wells. The relatively short distance to the sound or ocean indicates that the water has relatively short residence in the aquifer and little chance to mineralize to the degree that I might experience here in the Midwest.
I'm betting that the water quality is much more consistent than reported. A call to the water utility would confirm that. I expect that the average values they will give you are much closer to the upper end of the range. The low values are from when you folks get a deluge.