I don't understand how it's an example. It sounds like ssize_t and ptrdiff_t need the same number of bits on this architecture. And if ptrdiff_t is wrong, they'd probably make ssize_t wrong too. Also I don't think "we need more distinct constants because then if a compiler author deliberately makes one wrong they might leave the other one alone" is enough justification to have two.
ssize_t isn't standard C. It's just POSIX, which uses undefined behavior of the C standard to create an almost but not quite C language used on all Unix systems. POSIX C is not strictly conforming C, but it is conforming C. The same goes for GNU C, Visual C, etc.
