I do hope this is a wind up. If the individual hull variations are significant, the benefit of fleet weights is lost in the operational cost penalties. Any sensible operator would schedule multiple fleets for the purpose of fleet weights in such a situation. No one in their right mind would tolerate that sort of variation across a fleet weight mix. More importantly, fleet weight protocols will involve a maximum variation of an individual aircraft from the mean. For instance, in Oz, this is 0.5% which wouldn't permit the scenario which you describe.
Our fleet of 737s, -600 to -800, come from a host of backgrounds.. They have/had a variety of equipment installed such as airstairs and different galleys. The people at the back office has arranged them within different fleets, but even within an existing fleet we have tables for individual differences to DOW/DOI. Most are 0/0 but I took an example of one that has +660 kg on DOW. I think it complies with the intent of the 0,5 % rule (thanks for the number), and so does our CAA it seems.
If the accuracy of the loadsheet can be verified, to the best of our knowledge, correct registration or not, I am happy to go.
I have another one for you JT that you might find interesting. Due to custom regulations (and don't ask about the rationale) on our charter destinations we ferry tax free merchandise in the cargo hold as freight. At the oustation it's handed over to our passengers and for the return magically disappears into the standard weight of pax + hand luggage. I would guess it's regularly in the order of 500 kg.