Because the price paid by your utility for natural gas fluctuates from month to month due to free market economic factors. The utility must purchase the natural gas commodity every day to assure a constant supply for its customers. The price it pays is market based. That is, the price the utility pays is based on supply and demand for gas in the market place. The utility bills you for that cost through a Fuel Factor which is a Gas Cost Adjustment (GCA) or Gas Cost Recovery (GCR).

The cost of natural gas changes daily. Because of these fluctuations, regulatory authorities have created the GCA factor to insure that the actual cost of gas is passed on to consumers. Since the base amount on a consumer's bill does not include the cost of gas, the GCA is used by the utility to recover the actual cost it paid for gas during a particular billing cycle. Some utilities adjust this cost factor on a monthly, quarterly or semi annually basis. The longer adjustment periods may result in a delayed impact on the consumers' monthly bill during times of wide fluctuations in the price of natural gas.

Consumers' gas bills fluctuate monthly because of:
The amount of gas consumed in your home, and The Market price paid by your utility to supply gas to you for heating your home, heating water, cooking and supplying gas to other appliances.

Currently, neither the Railroad Commission nor any other governmental entity regulates the price of natural gas. The price is determined by competition. The companies that supply gas to your utility compete for your utility's business.