May I suggest a different version of this fairy tale?
Conductors ratings were and still are for the continuous load. A breaker would carry it's rated current with some margin forever. There was just one little problem: Thermal breakers act on heat. When you put a lot of breakers side by side, the breakers warm each other. Nice and cosy, but this made them trip at just over 80% of the load they are rated for. It was expensive making breakers that understood to compensate for the warm and cozy environment, so instead an 80% rule was introduced in the code.
Europeans who have always preferred undersizing hid the information about the shortcomings of breakers in the back in the breaker's information sheet. As long as there are "cold" breakers between the "warm", it doesn't matter.
Now there are temperature compensating breakers which means that 20A is always 20A. In the US you can sell them as 100% rated breakers.
