There's a difference between the minimum legal space for commercial chickens kept under intensive maintenance (including things like debeaking and forced-air ventilation), and the space needed for chickens to live a more normal, healthy chicken-life.

The Usual Guidelines aren't magic numbers, but they do work for most situations.
Sometimes a higher density works for various reasons (you will see in some hatchery catalogs the note that a given breed "tolerates confinement"), -- I currently have 5 chickens in a coop intended for 4 and they are doing well -- probably because I have a vastly-oversized run.
But if you read back through these forums you'll see that a majority of the problems people have with their chickens, both health and behavior, are either caused by or made worse by over-crowding and/or poor ventilation.
Here's a useful article about chickens' space needs and why the guidelines are *guidelines*, not *rules*:
https://www.backyardchickens.com/articles/how-much-room-do-chickens-need.66180/