There's nothing in the zone system that says the "real" film speed is 1/2 the box speed. The film speed is determined by testing. It might end up being 1/2 the box speed or it may not.
True enough. But isn't it a rather unusual "coincidence" that after testing, most people arrive at the same basic adjustment ?
I learned the Zone System from Fred Picker around 1970. Ansel had tested Tri-X + HC-110 and determined a film speed of 250. Fred did his own tests, and determined a speed of 250. I did my own tests and determined a speed of 250. It makes sense after all: we were all testing the same film and developer combination, doing the same basic agitation, with a decent thermometer, etc.