The average person needs far less than 65lbs (30kg) of water per day, which is a third of the average male weight in the US.
By medical standards humans need more like 8lbs (under 4l) per day for men and 6lbs (under 3l) for women. Less if your rationing and not exerting yourself.
The way to allow for extras is to store water for a longer length of time. If recommended value is two weeks, and it is in my area, then a month is a good buffer.
Also you need to clean your home - you don't want to get sick in a catastrophe scenario.
I was on a hike that recommended 1 gallon to complete it for 5 miles round trip.
I’m sure there is a method to the madness of 8 gallons per person that isn’t easy to see. There is a lot of rounding up with the military also.