Diffrent output of "ps -a" on Ubuntu and Fedora

I have noticed that the ps -a command produces different output when I run it on ubuntu "16.04" and Fedora 23. According the man page of both distribution the -a option should do :

-a Select all processes except both session leaders (see getsid(2)) and processes not associated with a terminal.

This doesn't seem to be the case for Ubuntu.

I am unable to explain this behavior as it seems that they have the same version of procps.

Fedora:

• Name : procps-ng
• version : 3.3.10

Ubuntu:

• Package : procps
• Version : 2:3.3.10-4ubuntu2

Have you any idea why this is happening ? Am I doing some thing wrong ?

Thanks, BR,