The reason this is impressive has less to do with the tolerances themselves and more to do with backward compatibility across decades at scale. That's the genuinely hard part.

The history here is deeper than most people realize. The United States spent fifty years (roughly 1800 to 1853) at the Springfield and Harper's Ferry armories trying to achieve what LEGO now does routinely: parts manufactured to tight enough tolerances that they are truly interchangeable without fitting. In 1853, a visiting British inspector randomly selected ten muskets made in ten different years, disassembled them, mixed the parts, and reassembled ten functional muskets using only a screwdriver. Tolerances of a thousandth of an inch. It was considered impossible by most of the engineering establishment of the time.

The way they got there was by building machines, then using the parts those machines made to build better machines, then using those improved parts to build even better machines. A virtuous circle of transferring skill from human hands to tooling. This is the actual origin story of what historians call the American System of Manufacture, and it's the foundation the entire modern automotive supply chain sits on.

So yes, any competent injection molder holds tight tolerances today. But that's precisely the point: the reason it seems unremarkable now is that two centuries of compounding precision made it so