Yes it is all about the bell curve of scene luminance range distribution. Not the actual for an individual shot today, but the average considered when the first light meters were made. Unbalanced scenes are not covered. They end up in bad exposure (unless you use a gray card or incident meter).
Originally Posted by Stephen Benskin
It's NOT in the middle. It is a full stop up from the middle. But wherever it is - it is there for coincidental, arbitrary or scientifically chosen arbitrary reasons.
The scale on the Weston Master III progresses
Which corresponds to
There are four stops beneath V and three above.