Very often, and especially during the "fuel crisis", a tanker unloads fuel into the forecourt and it's not necessarily the brand you think. It just meets the specification. I remember, decades ago, the House of Commons Select Committee discovered this during questioning and were shocked/surprised. I wasn't; I'd known this a long time (members of parliament often don't know what they are allegedly expert on). One thing that did change was a lot of TV ads stopped suggesting that their fuel made your car somehow better than other fuels.
The higher price "Shell V-Power" and "BP Ultimate" are more likely to be what they say they are, if that also helps aid restful sleep. The normal 95RON and standard diesel will usually come from their own stocks but the logistics people will often deal amongst themselves for whatever is the easiest solution. In the UK, my Focus' 95 RON can be from whatever source and it does not care (in Malaga, I use Petroprix for the same reason). For my DTi, I prefer £1 of Redex to go in every tank of gunk rather than pay 10p/litre more for V Power etc. I save £4 and have a sparkling clean fuel filter and old 5w30 when they are replaced.
to be fair, Peps on here started to use V-Power (I think, either that or BP Ultimate) in his 1.3 CDTi Combo and recording his distances on a tankful compared to regular diesel for similar journeys. By my maths, he was getting more km per l to easily justify the extra cost.