Actually, the numbers are very old and based in agricultural practice from before powered ventilation was available. This is a quote from an article I'm writing:
Where do these numbers come from?
Agricultural science has a very long history, though it wasn't necessarily applied to chickens until the early 20th century. The suggestion of 4-5 feet of floor space in the coop was made at least as far back as Poultry For the Farm and Home, a guide published by International Harvester's Agricultural Extension in 1921 with the goal of improving egg productivity by getting at least 100 eggs per hen per year from flocks of Leghorns. (Link to download .pdf here).
We can say that these numbers represent time-tested guidelines developed over at least a century by both scientific study and the practical experience of millions of chicken owners. They are, however, guidelines, not hard-and-fast rules. I suggest that you read this article to help understand why these numbers are only a starting point. Additionally, we need to keep in mind that these numbers generally represent minimums and that some situations might call for significantly more space.
Here in the Steamy Southeast, I find that I need at least double or possibly triple the ventilation to keep my coop under 100F when the temperatures are over 90F unless I can provide deep shade.
