I've noticed that this week. After spending a little time just reading on how to correctly write posix compliant code that was also guaranteed to do what I want, it's hard to come to any other conclusion than "posix is a last ditch attempt to slap a bandaid on Unix fragmentation by attempting to retcon some lowest common denominator behavior from a couple popular unixish operating systems and calling that a spec".

It's not what I would actually call designed. The closest analogy I can think of for non-c programmers is that posix is like if we decided that people should only make websites using javascript that was mutually interpretable by IE6 and Netscape navigator and we occasionally made updates every decade or two.

There's really nothing particularly noble or good or correct or elegant about all the API calls with implementation defined semantics. At best it's a necessary evil for compatibility. You can admire the cleverness required to get a single simple c codebase that works correctly on multiple operating systems and future operating systems that conform to posix in creative ways, but only in the way I admire those old school zines that are simultaneously a PDF and a jpeg and a shell script. Clever and ingenious but not actually good engineering design.