I think some times people underestimate how much walking there is in an average day when you're not sitting at home but walking everywhere. My wife recently took the grandkids to Disney World where her fitness app said she (and therefore. kids) averaged 7 miles a day, just on the Disney property. And that didn't include the hours of standing in line waiting. We were just in Dublin, where, on our own, we did 5.5 miles one day, just walking around the central tourist district. I cant imagine visiting any major European city which didn't involve walking a lot, unless you took taxis everywhere. I don't remember a lot of stairs, except for getting up and down to the U-bahn, or the usual steps in the hotels.
We did GAS in 2014, and the longest sustained walk was to our hotels in Baden-Baden and Mürren which was a few hundred yards. That was because the coach could not get close to hotel in B-B and of course, Mürren doesn't allow vehicles. Why the range? Well they don't always have the same hotels or sight-visit schedule, so actual distances vary. And sometimes you are on your own to get back to hotel. Some people assumed that the coach would be driving us directly to sights every day.
And then, I think they want to discourage people from overestimating their limitations.