Welcome to the AGL Community!
A standard gas bill is made up of two charges, your 'Usage' charge and your 'Supply' charge. As the name suggests, your 'Usage' charge accounts for all of the gas that you've used, which can vary greatly depending on how much gas you do or don't use. Your 'Supply' charge is charged to you at a daily rate regardless of your usage charges, and this covers all of the costs associated with supplying the gas to your property (costs of maintaining gas lines, etc).
Hopefully this clears things up!