Gross Weights

[Rex]

I have recently become a bit confused regarding the different Gross Weight certifications on Skymasters. In reading the history, the GTOW has varied significantly from less than 4200 to 4630. My question for anyone who may know the answer is this: Given the same type (i.e. non-pressurized, 337), if the design hardly changed over the years, is there any reason why the GW changes so much? Said another way, is a 70s model really stronger than a 60s model?