u flag with ps command According to the Mac man page for ps it says for the -u flag:

-u      Display the processes belonging to the specified usernames.


However, it seems to display different information rather than just being attached to usernames. E.g. here's the headers for ax:

PID   TT  STAT      TIME COMMAND


and here are the headers for aux:

USER              PID  %CPU %MEM      VSZ    RSS   TT  STAT STARTED      TIME COMMAND


Any idea why? Is the man page incorrect?
They're both sort of correct. There's a difference between ps u and ps -u. Consider:

$ ps -u blm
  UID   PID TTY           TIME CMD
  502  4278 ??         0:00.04 sshd: blm@ttys000
  ...

$ ps u blm
ps: illegal argument: blm
usage: ps [-AaCcEefhjlMmrSTvwXx] [-O fmt | -o fmt] [-G gid[,gid...]]
          [-u]
          [-p pid[,pid...]] [-t tty[,tty...]] [-U user[,user...]]
       ps [-L]


Also:

$ ps aux
USER              PID  %CPU %MEM      VSZ    RSS   TT  STAT STARTED      TIME COMMAND
root                1   1.9  0.1  2455856   4648   ??  Ss   Wed05PM   2:52.80 /
...

$ ps -aux
ps: No user named 'x'


The man page documents the options preceded by -, but ps (for compatibility with other Unix flavors) also takes options without a preceding -, with often completely different meanings.

In the manual page, go down to LEGACY DESCRIPTION for more information.