[einhverfr]

I wonder if it is an internal/external optimization. The whole point of metric is that it is built around the number system, leading to easy conversion between units.

The whole point of the American unit system is that they are optimized for specific use, but definitely not for conversion.

ISO paper sizes are a great counterexample, actually, because they took an application, rather than a conversion-centric approach. I.e when you print a signature you do so on a larger piece of paper, and then you fold, crop, and bind. Each fold cuts the paper size in half.

So ISO paper sizes are much more like American units than American paper sizes are.

[jacobolus]

ISO paper sizes assume that content is scale-invariant.

US paper sizes assume that you have specific design sizes to your printing technology (picas) and then give you a grid of an even number of those units.

Graphic designers / typographers I know tend to prefer the US paper sizes.

Regular people who want to make photocopies of enlarged/reduced pages tend to prefer ISO sizes.

[einhverfr]

Totally misses what ISO paper sizes are designed for, namely book binding and signature printing. That's why the scale-invariant approach is so important. Once you know how many leaves in a signature and what page size you want, it is easy to figure out which paper size you want to print on.