And that's not to say that every airline uses these standard weights. The 747s I dispatch (and do the W&B for) definitely don't. We weigh all the baggage at check-in and know the actual gross weight of all the cargo. So, the only "guesswork" is:
1) The "standard" pax weight of 160lbs which includes cabin baggage
2) The standard pantry weight is used based on the config
3) Distribution of baggage weight is based on how full each container is. The average checked-in bag weight is used to calculate the weight of each container. The actual weight of the container is not measured.
Although with the AAA baggage system I suppose it's possible these days to know how much each container actual weighs since you know (in theory) which container contains which bags. Although for a 747, I doubt it makes a hell of a lot of difference.
Can't speak for any smaller aircraft though....