It is scientific, but only partly. It has to include practicality, adherence, and every other human factor. They can’t say 2 miles because nobody could comply. They can’t say “2 meters minus the (square root of the outside temperature in Kelvin) in centimeters” because everyone would be flummoxed by the math. When getting a large group of people to coordinate an action, communication clarity/effectiveness is more important than the specific recommendation itself.