I just opened one of the 5 sealed copies I just picked up... I had a hunch.
One of the 5 weighed a little over 10oz. and the other 4 weighed a little over 9oz.
I check my opened copy with the thicker "bound" gameboard and it weighed closer to 10oz (no shrink so it was a little lighter)
So I opend one of the 9oz. sets to see if it in fact had the cheaper laminated board, which it did.
Also, the lighter games with the laminated boards tend to have more purple covers while the heavier bound sets have the slightly more blue covers.
Also, all of the lighter ones have the box bottom inserted upside down
FWIW.
My guess in the cheaper ones came later... the more purple boxes look just a tad newer in condition and print while the bluer ones were a tad more shop-worn. TSR probably had decided to cut corners to keep the game in print but to still keep it in the catalog... maybe to make use of the license till it lapsed or whatever.
Woud be great to get some insight on this one.