Must be an older version of ar.
Typically on MacOS X, the libtool -static command is used to create libraries, and it automatically updates the table of contents so ranlib is not needed. libtool also handles fat libraries better than ar; this was the main reason it was written at NeXT I think. Note that this is a completely different different program than GNU's libtool; NeXT's program predated GNU's and they've never changed the name. GNU's libtool is available as the glibtool command.
Since NeXT/Apple's development environment doesn't ever use the ar command, I doubt they've had much reason to update it.