There is a difference between over sold and over booked (from a post by DaveN)

Overbooked - More passengers have booked on the flight than there are seats
Oversold - More passengers have turned up for the flight than there are seats

Overbooking occurs with the expectation that a certain percentage of passengers will not turn up/cancel.
Overselling occurs when the airline has miscalculated the number of no shows/cancellations’