I am a country dweller. I go food shopping once a week and use a medium sized Waitrose in a town centre with lots of other shops and services, so that I can do everything from go to the dentist to have my hair done at the same time.
Visiting any other supermarket, would mean driving around the area from town to town and shop to shop, plus a town centre visit for other things and the car costs, not to mention time wasted, would far outweigh any possible savings in money by using a cheaper supermarket.
How much you 'save' (or not) by using one supermarket rather than another, depends on what food you buy and eat. These supermarket comparisons are based on notional baskets of food and you could go to the cheapest supermarket and spend more than at the most expensive, if you are buying things not in the standard comparison basket.
Supermarkets deliberately keep items in 'the standard food comparison basket' cheap, in order to do well in these comparisons, often choosing to loss lead with these products, but then choose to load the prices of items not in the basket. Prices will also vary from shop to shop in the same chain, depending on the area the shop is in.
I suspect that Waitrose are quite deliberate in their pricing to make sure their basket is fractionally more expensive. Doing this helps support their positioning of themselves as an upmarket supermarket, that people will shop in for the vicarious feeling of luxury living for very little extra cost. very good for business.