Difference between "ps -o" and "ps o"

In a script, I am trying to retrieve some details of all processes of current user on linux (RHEL 6.4 and 5.7). I was using "ps -o pid,args,euser" for it. But I realized it was not including processes owned by same user but launched in different shell session. Then I tried "ps o pid,args,euser", it works. In man page, I could not see anything which implies such difference. What causes it?

$ ps  -o 'pid,args,euser'
  PID COMMAND                     EUSER
13491 -/usr/bin/ksh               ak
13519 bash                        ak
25427 ps -o pid,args,euser        ak

$ ps  o 'pid,args,euser'
  PID COMMAND                     EUSER
13491 -/usr/bin/ksh               ak
13519 bash                        ak
13699 -/usr/bin/ksh               ak
13727 bash                        ak
20573 -/usr/bin/ksh               ak
20616 bash                        ak
20996 -bash                       ak
21027 screen -D -R                ak
24842 /apps/ak/localdis/pub/cpyth ak
25460 ps o pid,args,euser         ak
25714 -/usr/bin/ksh               ak
25742 bash                        ak

Solution

To quote from the man page (available here via first result on Google)

  ... The use of BSD-style options will also change the process
  selection to include processes on other terminals (TTYs) that are owned
  by you; alternately, this may be described as setting the selection to
  be the set of all processes filtered to exclude processes owned by
  other users or not on a terminal. These effects are not considered when
  options are described as being "identical" below, so -M will be
  considered identical to Z and so on.