A load is 'said' to be weighed before loading, sometimes at the airport, sometimes at the shipper's but in reality this is not always the case.
Whilst SNAM's view is understandable he probably wouldn't last for very long in the employ of many a cargo airline. Often the loading is done when the flight crew are still tucked-up in bed thus it is down to the loadmaster and if one name should 'stick out' for repeatedly delaying services, upsetting handling agents or peeing-off customers, well heads will roll.
Agreed this should not be the case but take for instance a departure from Nairobi, due to temperatures you only have a few hours each day whereas you'll get the aircraft off the ground and that's the same time as all the other cargo boys are going. You demand an offload & reweigh then you go right to the back of the queue, miss the window thus a 24hr delay.
Your load is fresh flowers, straight out of refrigeration, how are you going to explain 100T of dead flowers to your boss and the customer!
Some airline(s) have a reputation for operating overweight, on a DC8-55 it might have been 5T over, on a B747 perhaps 10T over. But, if there are 2 customers, each with a part load, and both believe they are the only one to add the extra tonnage then the situation may arise whereas an aircraft is 20T over.