The true dry weight has been required to be on a label for each individual trailer since 2009. If the dealer adds more than 100 pounds the law requires an additional label indicating the weight. The dealer can add up to 100 pounds without changing the label.
I think the OP is referring to the brochure weight which is used for marketing and is unregulated. The manufacturer can make up any number they want. I imagine each manufacturer has their own formula/method.
The law requiring each trailer to be weighed and labeled was a result of the wild inaccuracies of the brochure weight when compared to the real weight of the finished product.
