There is a procedure called a headcount whereby the number of occupied seats is tallied against the manifest (passenger list). From previous discussions here, some airlines do this others do not. Can't speak for AC, either they do and someone forgot or they just don't bother.
There's a story from many years ago in which a first-time flyer on one of the US regionals failed to deplane at her destination and a number of other intermediate stops, eventually ending up where she started several hours later (I'll see if I can locate it).