I think this happens at our place - the workers at Water Department skip reading meters sometime. They just write a number basing on the amount you used before. And then, at a month when they do read the meters, all the difference will be reflected in this month's bill. I had a bill before that made me think there was a leak somewhere. But after that month, it went back to normal. It happend again last year. One large bill in a year and back to normal thereafter.
Of course, they will never admit it.If you call, they will just simply tell you that you have used that much.
