asterisk users - Oct 2004 - queues.conf - Agent groups specified with : not @, difference in working?

>From the sample queues.conf:
 member => Agent/@1   ; Any agent in group 1
 member => Agent/:1,1 ; Any agent in group 1, wait for first
                      ; available, but consider with penalty

Can anyone elaborate on "wait for first available" and a scenario in
which
that might be used? I'm unclear on how it figures in to the big scheme of
things - is it applicable if your queue membership is only groups, or
multiple groups?

Surely if a call came in to a queue and all agents were busy, the first
available agent would get the call anyway? Or am I misunderstanding the
comment here?

Thanks in advance
Paul