When examining weights which cover many years of work, there will very likely be differences in quality from any maker (except, perhaps, for such as Perthshire Paperweights, where very strict, almost perfectionist, rules of the main lady scrutineer were applied).
My understanding is that when JD was producing the J Glass weights, special care was applied to each weight to ensure that it would compete in what proved to be a very tough market for the best quality items. I think today's thinking is more about making a living than trying to be the best all the time.
I don't find it surprising that current weights show a range of quality, particulary those produced at the cheaper end of the market.