The airline's inventory system reports 'x' seats saleable to the distribution system or agent. 'x' will invariably include a percentage to allow for no-shows, misconnections, etc, although as someone else said the percentage should be pretty low on that type of flight.
The distribution system will book 'x' bums on 'x' seats. If 'x' was incorrect and resulted in bumping then it is the airline that is responsible.
The airline has systems or people to make the decisions regarding overbooking and mostly they get it right. Sometimes they get it wrong, and sometimes the number of seats changes due to circumstances unforeseen or beyond the control of the carrier, for example MTOW restrictions, a configuration change, having to accommodate passenger who misconnected from an earlier flight, and so on.