United States, Burma & Liberia

Initially, it's an intriguing question - then after that, you kind of feel stupid for thinking so. Well, in the first place - a universal system was made (in this case, the metric system) where one millimetre was this much, and so on. The question is how this system was spread. Did some random guy go up to random guy #2 and go:

Random guy #1 - "Hey, there's this system I made for measuring. This is one centimetre *offers a scrappy piece of wood*"

Random guy #2 - "Okay. I'll take this scrappy piece of wood to my village, pass it onto other villages so that we build up a multi-synergetic community of measuring systems."

Random guy #1: "Thanks. Bye."

They both ride off into the distance (or walked away in this case).

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I guess that measurements are so ubiquitous that we use existing rulers to make other rulers. And on the software which is used to make the templates for rulers, there are already established programs with the system on that.

Everything is arbitrary.