What you need to understand is the fact that airlines will oversell flights on a regular basis since they know that statisticaly a percentage of booked tickets are no-shows, standby on another flight and so on.

On a plane with 100 seats, they may try to sell 120 seats, counting on 20 not to be there at the time of departure. If 105 show up, they just ask for volunteers or they will bump someone off to another flight. Happens all the time. They would rather do that than leave with 20 empty seats.

The seating charts are often inacurate since they may not show elite and/or exits seats available. So the chart may under estimate the number of seats available.

I hope this helps. Bottom line for your flight - likely very busy!