Incorrect meter reads
 
Each meter reader reads hundreds of meters each day. Mistakes are rare, but happen occasionally. Sometimes if the meter reader can't see the meter it will be an estimated read. In either case the read will be corrected on the next actual reading of the meter.

The kilowatt hour reading on the meter is continuous. That way if a error is made on this month's read it will automatically be corrected next month. For example: if the meter showed 14,000 kWh last month and it showed 15,000 this month, but the meter reader recorded it as 15,100 you would get a bill for 1,100 kWh instead of the 1,000 kWh it should have been. Next month the meter might show 16,000 and when that is recorded properly you will be charged for only 900 kWh instead of the 1,000 kWh that was actually used. You always get charged for the difference between the current read and the last read. The meter does not reset to zero and start over each month.

Meter reading errors are self correcting over time. You can learn more about meter reading here at aps.com.