I always keep my eyes peeled for overbooked flights and my experience is that . . .
I think different gate agents or even phone reservationists look at different numbers on their computers - some of which may include, or NOT include - non-rev's, standbys etc.
One time, I was told that a flight was overbooked. I offered to volunteer. She said "GREAT"!! Then another agent walked up. The first one told the second one that she had a volunteer since the flight was overbooked. The second agaent told the first one: "this pax is on the later flight and is on stand-by for this flight. We don't bump people off for stand by's." It turns out that the original agent who told me that the flight was overbooked was a new agent, just out of training, and wasn't quite familiar with all of the procedures yet! It looked overbooked to her!!!! Oh well!!