The actual amount of water each customer uses is highly dependent upon personal needs.  It is clearly true that two families of four, even neighbors can use considerably different quantities of water in a single month whereas the average for many families of four could be fairly consistent.  There are several factors that account for varying water use.

Weather plays a big factor in consumption.  Some customers have electronic irrigation systems that run even when raining while other customers use manual systems and closely watch the weather.  Also, long periods of hot, dry weather ten to turn the grass brown and individual customers vary in their desire to address that issue.