Surely, you start with what your income is prior to retirement, i.e. wages, as a starting point. You won't need more than that if you've been living on a certain amount for years/decades (unless you plan major changes).
As others have said, the figures are prepared usually by pensions firms, so they have a vested interest in trying to pursuade people to save/invest more.
Some costs will reduce, some will increase when you retire. You won't have commuting costs to work, you'll probably have paid off your mortgage (lots retire when they pay off their mortgage as it's usually the biggest cost). No more work clothes to buy, snacks/drinks/lunches during the working day. But, you'll probably spend more on hobbies/entertainment, etc. It's all personal to you!
Spending may well be higher in the first years of retirement as you do things you've not had time for, i.e. travelling, home improvements, etc., or buying "One offs" like a new car, caravan, moving home, camper van, "Once in a lifetime" holiday etc. But you're not going to be doing those every year - they're one offs.
OH and I are currently living on £25k per year (net) between us. We're 7 years away from retirement. No mortgage, no expensive hobbies, "normal" holidays rather than exotic long haul ones. It's a perfectly good standard of living. We have lots of savings but never use them, so we have a good backup for when we need to buy a new car, home improvements etc - don't need to use income for that kind of thing.
We're more than comfortable to plan to live on just 2 x state pension which is about the same £25k that we live on now. We literally "couldn't" spend the kind of income that some of these reports suggest - it'd mostly just go into our savings, certainly not "needed".
People really need to do their own sums and make decisions based on their own needs rather than being fooled by biased reports.