you're comparing retail prices to wholesale prices. NG may sell the power on Block Island at 9.5c/kWh for the generation component, but it is not able to procure is so cheaply at that particular location.

Also, the 24.4c/kWh will be for one of the supply sources of NG and will be mixed with its other sources. It is NG that will bear the possible additional cost on a small fraction of its supplies (possible, because the rest may become more expensive when gas prices go up again, as they will).

Wind power