Vague thoughts, for what they're worth.

Mould blowing is a highly skilled process, perhaps the difference is because of the workers gaining more skill at making these?
It starts off with somebody having to cut
exactly the right size of glob of glass from a gather, and given it's a substance the texture of runny honey, that really isn't an easy thing to do.
The weight of the final pieces will depend on the weight of the glob cut.