Hehe.. you are thinking in terms of a ‘cost plus’ pricing model. That is, you charge what the thing cost you plus a bit for your trouble.
Most things that sell on a large scale are not priced this way. They charge as much as they can before sales start to drop off too much. So for example, in a town there would be lots of petrol stations competing for business, so prices will be driven down. If you’ve got one petrol station on one side of the road then another on the other side HAS to charge the same otherwise absolutely no-one will go there.
If however you’ve got a petrol station on the bypass a couple of miles from town, you can charge more since loads of non-local people will be driving past and fill up there without knowing that it’s cheaper in town, or not considering it worth it to leave their route.
There will be factors influencing why the other tesco is cheaper – you could probably figure it out if you fancy a puzzle.