Each bag is weighed individually. But that doesn't necessarily mean that data is transferred to the Dispatcher. Not all airlines work on the real weight of the bag, but an assumed weight as nclops says. That assumed weight is what the Dispatchers loadsheet is based on. Some airlines do work on the accurate weight, which is added to all the other accurate weights and divided by the number of bags in total to create a more accurate average weight than the airlines own assumed figure.
The reason bags are weighed individually is so that the check-in staff can be certain that it falls within your allowance, (and can calculate the excess baggage fee if it doesnt) and/or so that they know wether it needs a HEAVY tag to identify it to the baggage team.
Another reason they may ask about boot bags/skis etc, is that some airlines like sporting equipment to be placed in a separate hold or container.
RTG!