Do home radiators waste water?
To answer your question, the radiators in your home are a closed loop. If they are single-pipe steam radiators, then there is a small amount of vapor loss out of the little valve on the radiator. (The one that may hiss a bit while warming up.) If you have that type of system, the water level in your boiler must occasionally be topped up, but that would be a few gallons of water per month. But if your heating system was installed in the last few decades, it's more likely you have the much more common forced hot water, which is an entirely closed loop. 

My guess is that your meter was misread, or the meter is faulty.