Producing 10,000 glass with a 10g reduction is 100 kg... and the amount of cost saved per day would be significant without greatly compromising the design. I would expect such economy became important after the end of communism and the need to increase profitability became important. Most of the Sklo Union designs are extremely heavy compared to glass produced in the West in the same period. But that is speculation!
It may also be the measurement of the parison was made with a loose tolerance and the mould was arranged to accommodate different volumes of metal by the depth of the plunger. Without knowing what machines were being used it would be hard to say... but perhaps one of the workers that are still around can be contacted to answer this question. Hand operated presses might see largish variations between teams and semi-automatic presses less variation being likely.
It was a phenomenon that I noticed a lot while appraising the collection, where apparent pairs did not match in various ways. This was why I split these apparent pairs/sets - as they may well have been produced in batches years apart only to be re 'setted' by a collector. I would expect the variance in weight and dimensions to be much smaller in items made at the same time. Many designs were made over long periods and for production volume as well as wear and tear on the moulds and plungers would have seen moulds repaired or new moulds made.
In some cases the variations can be quite dramatic and could even be deliberate adjustments of a design to suit the changing market taste over a long production period. For example Jitka Forejtova 13171 Nude figure http://sklo-union-glass.com/index.php?page=shop.browse&category_id=13&option=com_virtuemart&Itemid=1
311371 shows at least 3 distinct moulds in the examples appraised. As the size of the breasts change that could well be a deliberate change for the market. It would interesting to find a dating for these type of variations but very hard to achieve without dated photographs. But such aspects do make for an interesting study of both manufacturing and marketing aspects with their impact on the design of the SU glass.