You are right about it coming from the same refinery,
I Work in the oil and gas sector for BP, I used to work at the Grangemouth refinery which supplies the whole of Scotland and north England with fuel. Shell , Tesco, Sainsbury's, ESSO and Texaco all these companies buy their fuel from this refinery. All they do is when the leave the refinery gates put in some additives which mix in the tank en route to the forecourt's.
Another thing to Note, when buying fuel make sure you buy from a forecourt which isnt right next to the refinery as the additive hasnt had long to mix in the petrol tanker!