I think it goes by the address reported by the merchant. When I use the BP station in Porchester (Portsmouth), on the map it appears in the middle of Fareham.
I’ve used my American Express card, which provides merchant addresses, in there before and the address is reported as Fareham - same place as it appears on the map in the Monzo app. Must be their registered office?
When I use my Amex card in the local BP in my village (near Romsey, just outside Southampton), the address displayed in the Amex app is Wembley - again, must be their registered business address. I’m fully expecting that when I use my Monzo card there, it’ll appear on the map as some place in Wembley. I’ve seen crowd sourced corrections mentioned for the future - that would be brilliant - or maybe using the phone’s GPS (in a user instigated way, not a creepy automatic way)?