This is a limitation of the true type font format. The original naming only consisted of the "Family Name" and "Subfamily Name" fields. The true type format was designed very short sighted from begin, so they put a limitation in the standard, stating that each family must never have more than four subfamilies.
In the screenshot above you see the fields "Family Name" and "Subfamily Name" (red box), which are the old naming scheme of the true type format. Here you have the limitation that for each family there can maximum be four subfamilies.
Later they realised, that fonts can actually have more than four styles and also have weights etc. So they extended the standard. Because some operating systems (Windows...) relied on the four subfamily limitation, they just added a new pair of fields, the "Preferred Family Name" and "Preferred Subfamily Name" (green box).
Contrary to the old names, you have no limitations in the "Preferred..." fields. A family can have as many subfamilies as required. But to provide compatibility with older systems, you also have to provide a compatible naming in the original fields. Modern operating systems only use the preferred names.
Here a naming example for the Nexa Family:
As you can see, for the legacy fields "Family Name" and "Subfamily Name", I create several "small families" only with the different styles, while I create one big family in the "Preferred Family/Subfamily" fields with all styles and weights.
You will find more information here: https://en.wikipedia.org/wiki/TrueType
There are also some flags inside of the fonts which are currently not shown in font adjuster. They define if a font has a bold or italic style. The operating system is using this information sometimes to do additional grouping of fonts (Windows does this).