I have questioned this aswell. Working for a handling agent at LGW over the summer, the majority of the charter flights we checked in were 'pieces only'. (i.e. when entering pax details into the system, only the number of checked in bags is entered) The bag weight, I believe as told by the dispatchers, is based on an average weight, which i cant remember exactly what is is. Therefore, total number of pieces on flight x av. weight of bag = total baggage weight.
Some airlines however, do a 'pieces and weight' system where both the total number of bags and total weight of these is included in the system per passenger.
Which method is used is the airlines choice.
As for hand lugguage, this is the biggest problem as people seem to want to take those small wheely suitcases on board. Some of these weigh 5kg+ just empty. Why people check in suitcases then want to take all that stuff on board is beyond me.
When doing the gate for a Ghana Airways flight (747-200) we collected an extra 800kg of bags from pax arriving at the gate when told at check in they can only take 1 bag onboard.