On Thu, 5 Nov 2020 at 11:57, Henri Menke <henri@henrimenke.de> wrote:

%D The default dimensions are quite old and will not change. The funny fractions
%D were introduced when we went from fixed dimensions to relative ones. Since
%D \CONTEXT\ is a dutch package, the dimensions are based on the metric system.
The
%D asymmetrical layout is kind of handy for short quick||and||dirty stapled
%D documents.

https://github.com/contextgarden/context-mirror/blob/343fdd99ef79884fca1f86b49c950e03939aeedf/tex/context/base/mkiv/page-lay.mkiv#L1373-L1377

So it's for backward compatibility then.

Thanks for the nice anecdote!