The camera and camera lens systems evolved with photographic plates and film. The f/stop system, is based on a doubling or halving of the exposing light. The goal of the film and equipment maker is to allow the user to make a faithful image. Photo materials do deliver a faithful image when they are properly exposed however it is rarely necessary to hit the exposure spot on.
Film makers strive to make film according to a recipe that delivers the published sensitivity (ISO) but manufacturing disparities are always present thus the goal is to deliver a film that is + or – 1/6 f/stop. Often the real target will be 1/3 f/stop.
The lens maker builds an iris diaphragm adjuster into the lens barrel. You set the f/number via a mechanical linkage that adjusts the working diameter of the aperture. This is a gear linkage with backlash; the likely precision is about 1/3 of an f/stop. Suppose a 50mm prime is set to f/11, the working diameter will be 4.5mm. Now stop down 1/3 f/stop to f/13, the working diameter is 3.8mm. A exact and repeatable change with an accuracy of 1/3 of an f/stop is a mechanical nightmare.
Same with the shutter speeds, these are stopwatch type escape mechanisms. The between the lens shutter must open, stop, then close. The efficiently is not quire 1/3 f/stop. The focal plane can be highly efficient; it is only required to make a smooth pass over the focal plane. The accuracy of the slit width is better than 1/6 f/stop.
Film developing is a chemical process effected by time, temperature and chemical strength. The best a precise laboratory can maintain is 1/6 f/stops.
We add all these variables together and the likelihood of an accurate exposure + or – 1/stop is not good. Fortunately all films have latitude. The beauty of the positive / negative system is: exposure errors made in the camera and via film processing can be offset by adjusting the print exposure an or the type of paper and its processing. Bottom line, it was not necessary to make the exposure spot on. The slide film had less latitude. Later models of lenses and cameras attempted to provide ½ and 1/3 stop adjustments.
I managed a plant facility for many years that made test films, both negative and slides and prints. These were that were used to calibrate high-speed photofinishing printers and film processors. Our goal was 1/6 f/stop or better, it was daunting.