changes in hitch weight in the 70s

[transy416]

I've been spending some time looking at the trailer specs pages of 70's 31 foot models on Airstream's website. The hitch weights range from 545 lbs to 730 lbs across the years, being lighter in 1978 and 1979. I've been looking at center bath models and the floorplans look relatively the same over the years. Did the weight of the materials vary that much?

1973 695 lbs and 730 (twin bed, double bed w/center bath)
1974 650 and 690
1975 650 and 690
1976 650 and 690
1977 710 and 715
1978 545 and 545
1979 545 and 545