DL has policy of same day standby, so rev pax come and go frequently. I think it's very cheap to use it--maybe $50. Business people will do that often since companies pay for it. Maybe OALs have similar system.
Also, nonrevs have a bad habit of listing on multiple flights and not cancelling when they get something better. Misconnects might drop off, too, if no chance to get them there and they get rescheduled.