Why gas is cheaper at Walgreens

I stopped by the Walgreens in my hood (2499/3040) tonight to pick up a gallon of milk. When I returned to my car, I noticed that Walgreens has changed out its propane rack and the prices are much lower than before.

Last time I bought a tank of propane for my grill, I bought it at Walgreens. They were selling Blue Rhino brand gas. A tank exchange cost $20 or $21. Now they're selling Walgreens-brand propane for $15 for a tank exchange. That's a 25% price cut, for what I perceive to be an identical product, even with the brand change.