Groups with Runlevel4 in Slackware_14.0

When I use startx after a console login to Slackware_14.0,
I find that by default a user belongs to the following groups:

users lp floppy audio video cdrom scanner

When I alter /etc/inittab to start runlevel4,
the only groups a user has are:

users

The other groups can be added with "usermod" but why are the groups missing when booting with runlevel4, and which groups should be added?

This is a change from the way Slackware_13.0 worked.

Is this a bug in Slackware_14.0?

Bug? No.

See /etc/login.defs

# Note that users are added to these default groups only when
# logging into a shell with /bin/login, not when using a login
# manager such as kdm.  In that case, users who should have
# hardware access must be added to the appropriate groups
# when the user is added with adduser or useradd, or by editing
# /etc/group directly, preferably using "vigr"