linux - Why is the output from `groups` different from `groups user` if Im currently logged in as user?

Im not sure why the following have different output. My understanding is that groups without specifying a user give all the groups the currently logged in user is a member of.

jacob@box:~$ groups
jacob adm lp dialout cdrom plugdev lpadmin sambashare

jacob@box:~$ groups jacob
jacob : jacob

Also what does it mean "groups of the current process" (from the groups man page)? Where are they set up?

NOTE: My distro is Ubuntu.