Nicor Gas only makes actual meter readings every other month. In between the actual readings they make an estimate and bill you according to that estimate. It seems to me that when natural gas commodity prices are falling (as they are now), Nicor makes every attempt to over-estimate your usage so as to get you to pay a higher price for the gas. When prices are rising, however, they underestimate your usage and then you must make it up at the next actual reading after prices have crept up a little higher. This last month their estimate was about 25% higher than what I actually used. I read the meter one week later than their estimate and the reading was still less than what they estimated. I called in to get a corrected bill and the representative was very rude and tried to tell me that it would not benefit me to do this. I insisted and she finally changed the bill.