No-Shows are not only passengers who don't show to the flights. As a trick to reduce cost , travel agents often reserve round- trip flight even when the passenger is flying one-way. The 2nd segment is a "dummy" one and the airline get a "No Show" for a passenger who never intended to be on the flight.
Also , when a flight is overbooked , usualy there are seats available in Buisness and first that are cleared only at the airport so preffered passengers would be upgraded to clear space in coach.