Many changes in length of the models too .
Just perhaps , issues with the 2018s lead to Northwood, changing frame manufacturers ( suppliers ) even changes in slide drive mechanisms could account for some of the weight differences.
Tire size ; axle ratings; items that were popular options , now standard equipment?
You'd need to go over the entire build sheet and check each item .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.