answer:Their rate may have gone up, you could have a leak somewhere in your lines. I’m not sure if water companies do this, but I know power companies do, they may have estimated your usage either this month or in the previous months. To save time on sending people out to check meters, power companies will “estimate” how much power you used based on how much you’ve used previously, how much other people with your size apartment/house use (or previous residents at your house have used). Later when they send someone out to actually check the meter, they adjust their estimate and either credit you the overcharge, or charge you more to make up for it. IF water companies do this, it could explain your differences.