Why is the aperture ratio of the focal plane like that?

The aperture scale of focal plane is t value, also called "exposure series".

Different from SLR lenses, in still photography, it is not important that two lenses with the same specifications have different effects. Anyway, if you want to achieve the same brightness effect, you can make up for it by increasing the exposure time. However, when recording a movie, I'm afraid it's not that simple. After all, the film is continuous shooting, and sometimes the lens needs to be changed, so the light effect between the two films should not be too different, otherwise there will be light and dark deviation.

Then another reference value appears to measure the brightness of the actual exposure of the lens, that is, the T value that will appear on the name of the focal plane, which is also called "exposure series".