Originally Posted by
DawgmanOH
Lots of people have not boarded flights for which they have checked in for a variety of reasons. Heck, I've seen people stand up and leave a flight they were already seated on in an airplane.
Just because everyone has checked in, does not mean that everyone will board.
And especially with 24 hour check-in means that people checkin and don't make it to the airport for one reason or another. THere's no way to know if the plane is full or not until after they've boarded everyone waiting to board.