I have to agree that not informing you an your fellow passengers that your bags have been left behind was wrong. I am suprised that there wasn't a nosey passenger on board that spotted the bags coming off the aircraft.
I have been in similar case in the past where the crew have decided not to inform passengers at the other end or during flight in case they kick up a fuss and actually leave the handling agents the other end to break the news.
Yes passengers book bags in advanced however it is in individual passengers reservations as then they have the option to check-in the bag or not. It is not practical to go through all passengers individual reservations for every flight and work out if their male, female, child and if they have any bags booked.
Once the passengers check-in the system automatically breaks the passengers into Males, Females, Children & Infants including the number of bags checked-in in the system.