It's high because when they're all packed in there avoiding the snow or icy cold winds, they'll start getting cranky and that can lead to behavior issues.
I built for 4 sq ft per bird (though currently sitting at about 8 sq ft per, as I'm not at max capacity for the coop yet) and yes there's very stormy days where they really do not want to go outside for more than a minute. During periods of snow I clear out space in the run just so they'll come out instead of huddling and bickering inside the coop. Thankfully we get relatively few of those days - if you can manage things so that they don't end up staying indoors on days like that, then you can fit more birds than what I suggested.