If there is logic to how Word works, it is to some extent luck, IMHO.

There have been many hands in its development, many of those belonging to people do not use the program themselves, and many in the marketing end.

The way these particular settings work also was changed from Word 97's methods to the current methods. The way these settings work does make sense if you understand the underlying rules. If it is important, just remember that in earlier versions of Word, the settings are cumulative.

I'm not sure when they added the (useful) switch to not use space-between formatting for paragraphs in the same style.