Opening a gas station is a lot easier than acquiring mineral rights, drilling an oil well, refining that oil, and getting it to market. Oh, and your customers can't just drive across the street to your competitor because they are 1c/litre cheaper.

There's naturally going to be a lot more friction and a lot less pop-up competition and therefore a lot more margin on the supply side of things.

The station has no power to raise margin - they are in tight competition with every other low-margin station around them. The suppliers, on the other hand... If they invested into wells that aren't affected by the war 10 years ago, and their competitors haven't (or have, but can't supply all the world's oil needs), and there's a global supply shortage - they have lots of room to raise prices.