It is NOT complicated.
Either the designer got his weight estimate right or he got it wrong.
A good designer will look at three weghts. I know. I do this.
Weight one will be the weight "out of the box" or as the boat is delivered with no gear and no liquids aboard.
Weight two will be ( in my office) the weight of the boat in an average loaded condition, i.e. as you might find it any given weekend during the sailing season. This will include some gear, some crews affects, full sail inventory and some liquids.
Weight three will be the loaded condition, i.e. full crew weight with their gear, full tanks and full food and beverages, the way you might find the boat ready for a three week cruise.
I choose to design the boat to weight two, the typical way you will find the boat duruing the sailing season. I call this the "displacement". For me it seems representative and accurate.
You misunderstood what I said. What is complicated is not the different basic weights of a boat but to know to what weight the builder is referring when they give the boat displacement (they don't say what type of weight they are talking about). As Anders pointed out regarding the RM 1060 that is quite common several weights being given by the same builder in different occasions. With the Bavaria 36 (2003 model) they started given 4.600kg then 5.600kg and in some cases 6000kg.
"Lightship" is only useful to the designer and builder to check and see if their target weights are on. The final displ will vary with each owner and the gear he carries. The builder can only guess at that. You would have to get actual freeboards and a Pounds per inch immers ( or Euro equiv) to figure out the final displ. If you gave freeboards to the designer he could give you a very accurate displ.
Given all these confusions and since weight is an important promotional advertiser and Brands are interested in given to the public the lowest possible weight the RCD stipulated a standardized type of weight that is called Minimum sailing condition (MSC) that should be measured the same way in all boats, including the same percentage of fuel and water, sails,oil and everything the boat needs to sail.
The problem is that even if the correct MSC as the MLD are on the certification papers the builder is not obliged to give the MsC has the weight of the boat on the promotional papers and many refer the lightship condition or even the designer estimated weight.
That's what I mean with complicated, not the different weights but to know of what weight they are talking about.