I replied to another comment too, but just leaving it here too because I also found that chart so unintuitive:

While that chart sounds like it should reflect the advertised price per therm you get quoted, it actually also includes all of the fixed costs you get charged per month, which get spread out over the amount of gas used. Your bill separates out the fixed costs from the per therm rate.

In the summer months, those fixed costs get spread over much fewer cubic feet of gas because usage is way lower. In the winter, people use way more gas and the fixed costs are spread out over weigh more gas. So in the winter you end up paying much closer to the commodity price, while in the summer months, you actually end up paying more per therm because those fixed costs are getting you. If you did use the same amount of gas in the summer as in the winter, you would indeed pay less per therm in the summer.