I bet the discrepancy you sense is the result of them buying in advance, on contract. While the "spot" market drops, they may be on the hook to pay the higher price they promised. The reverse also happens - spot prices rocket, and their contract price doesn't (not immediately at any rate).
That would be my guess. But that was what I tried unsuccessfully to find out.
Interestingly, I received a bill yesterday ($10 a month even without using gas). And it seems that since I used no gas, I am not privvy to the current cost computations. Makes me want to run my heat for a few minutes so I can get a bill showing that information.
Question is, at what price are they (right now) contracting for delivery this winter?