It's a recommendation, and I'm confident that most people will adjust as needed. I don't track my fluid intake at home, but my wife and I go to Burning Man every year, and we each take a 2-liter container of water with us whenever we go out. For our day's consumption, we each drink at least 2 liters of water, plus coffee and juice over the course of a morning; soda, tea, water, or juice at lunch; at least another 2 liters of water each in the afternoon; more soda, tea, or juice with supper, another liter or so of water in the evening; more water before bed, and maybe wine or port or rum and coloa during the day or before retiring. We also eat canned fruit and drink the syrup as snacks during the day.

And that's just drinking. We also use water to cook, and we may bathe in water on an occasion or two. We do not do laundry while we're there.

Drinking water drops way off when we get back home, of course, but then we're doing laundry again and using flush toilets. (And daily baths.)

Water requirements are way understated if they include only drinking water and even if they include drinking and cooking water. Laundry, bathing, and the toilet never seem to be taken into account. If you've got no water flowing from the hydrant, you can't flush your toilets unless you have access to water from another source (pool, nearby pond or creek).