Many towns like mine, have a flat rate for water, which may be the case where you live.
Our town charges you a flat rate of 84 dollars, quarterly, which includes a 2 bag garbage pickup every Tuesday and 15,000 gallons of water over a 3 month period. If you use more than that, then their rate kicks in on top of it. Which, I don't know what that is because we've never even been close to going over that with 2 people living here. I don't need much RO/DI either.
If I was gone for 3 months and didn't use any water or garbage service, I would still be charged 84 dollars.
Your place may have the same deal where they're only charged quarterly and he's really asking for 13 bucks a month. Most places, water is very cheap.