27/64 is just a silly measurement!
No it isn't. It is just another example of what we call reality.
Imperial measurements have been in use for hundreds of years and worked perfectly well. For the USA and many other countries they are still the standard.
Metric measurements were developed separately in Europe and also work perfectly well in most European countries.
In the 1970's the UK adopted metric measurements (sort of) and now we are left with a mix of both, not only because some people prefer one system to the other but because we import stuff, including pen kits, from countries that still use imperial measurements.
Alongside this engineers, for reasons that I do not fully understand, developed letter and number systems of drill bit sizing. Perhaps an engineer could explain the reasons for this. Maybe like accountants they just like to make things complicated to confuse the uninitiated.
This is not going to change. the Europeans are not going to revert to Imperial measurements and the USA if it ever adopts the metric system will likely do so slowly and piecemeal just as the UK has done. The two systems will continue to exist sided by side long after any of us are around to worry about it.
Like it or not pen bush sizes have been designed to fit different kit dimensions by different makers over time. Some are Imperial some metric and some in odd letter and number sizes. This is also not going to change, especially not retrospectively. This whole debate has been entirely pointless and has achieved nothing, which is precisely all it could ever have achieved.